**7. Types of drug carriers in medicine**

#### **7.1. Polymeric nanoparticles**

Polymer-based nanoparticles are submicron-sized polymeric colloidal particles in which a therapeutic agent of interest can be embedded or encapsulated within their polymeric matrix or adsorbed or conjugated onto the surface [59]. The drugs may also be sensitive to gastroin‐ testinal degradation by digestive enzymes. The advantage of using polymeric nanoparticles is to permit encapsulation of bioactive molecules and protect them against enzymatic and hydrolytic degradation [60]. Therapeutically used polymeric nanoparticles are composed of biodegradable or biocompatible materials, such as poly (ε-caprolactone) (PCL), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), alginic acid, gelatin and chitosan [61-64]. Polymeric nanocarriers (NCs) may suggest an opportunity to target chlamydial organism within the contents, as NCs have been shown to be excellent intracellular carriers, and can be appropriate to encapsulate a variety of therapeutics containing biomacromolecules. Compared to free drugs, polymeric NCs have many other advantages including improved drug bioa‐ vailability, high carrier capacity, the ability to release the payload in a controlled behavior and to adapt to different routes of administration and to concentrate in inflammatory and infectious locations by virtue of their enhanced permeability and preservation. Conjugating NCs with specific moieties have also been shown to enhance their targeting to specific cells and tissues [65]. Polymeric nanoparticles have been extensively explored as means for drug solubilization, stabilization and targeting [66]. Polymeric nanoparticles possess several unique characteristics for antimicrobial drug delivery. Firstly, polymeric nanoparticles are structurally stable and can be synthesized with a sharper size distribution. Secondly, particle properties such as size, zeta potentials, and drug release profiles can be accurately tuned by selecting different polymer lengths, surfactants, and organic solvents during the synthesis. Thirdly, the surface of poly‐ meric nanoparticles typically contains functional groups that can be chemically changed with either drug moieties or targeting ligands [67]. For targeted antimicrobial delivery, polymeric nanoparticles have been repeatedly ornamented with lectin, which is a protein that binds to simple or complex carbohydrates present on most bacterial cell walls. For example, lectinconjugated gliadin nanoparticles were studied for treating *Helicobacter pylori* related infection diseases. It has been found that lectin-conjugated nanoparticles bind specially to carbohydrate receptors on cell walls of *H. pylori* and release antimicrobial agents into the bacteria [30, 67]. Rifampicin-loaded polybutylcyanoacrylate nanoparticles have also shown enhanced antibac‐ terial activity both *in vitro* and *in vivo* against *S. aureus* and *Mycobacterium avium* due to an effective delivery of drugs to macrophages [68].

#### **7.2. Hydrogels**

antibody-mediated active targeting, while the intracellular delivery could be mediated by specified ligands or by cell-penetrating peptides [49-53]. The purpose of drug delivery is to carry out sustained (or slow) and/or controlled drug release and therefore to improve efficacy, safety, and/or patient comfort [54]. Thus, the use of drug delivery systems has been suggested for passive targeting of infected cells of the mononuclear phagocytic system to enhance the therapeutic index of antimicrobials in the intracellular environment, while minimizing the side effects related with the systemic administration of the antibiotic [55]. These systems propose many advantages in drug delivery, mainly focusing on improved safety and efficacy of the drugs, e.g. providing targeted delivery of drugs, improving bioavailability, extending drug or gene effect in target tissue, and improving the stability of therapeutic agents against chemical/ enzymatic degradation [56]. The nanoscale size of these delivery systems is the basis for all these advantages [57]. It is therefore assumed that, DDS with enhanced targeting property is highly promising in increasing the efficiency and efficacy of therapy while at the same time

Polymer-based nanoparticles are submicron-sized polymeric colloidal particles in which a therapeutic agent of interest can be embedded or encapsulated within their polymeric matrix or adsorbed or conjugated onto the surface [59]. The drugs may also be sensitive to gastroin‐ testinal degradation by digestive enzymes. The advantage of using polymeric nanoparticles is to permit encapsulation of bioactive molecules and protect them against enzymatic and hydrolytic degradation [60]. Therapeutically used polymeric nanoparticles are composed of biodegradable or biocompatible materials, such as poly (ε-caprolactone) (PCL), poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), alginic acid, gelatin and chitosan [61-64]. Polymeric nanocarriers (NCs) may suggest an opportunity to target chlamydial organism within the contents, as NCs have been shown to be excellent intracellular carriers, and can be appropriate to encapsulate a variety of therapeutics containing biomacromolecules. Compared to free drugs, polymeric NCs have many other advantages including improved drug bioa‐ vailability, high carrier capacity, the ability to release the payload in a controlled behavior and to adapt to different routes of administration and to concentrate in inflammatory and infectious locations by virtue of their enhanced permeability and preservation. Conjugating NCs with specific moieties have also been shown to enhance their targeting to specific cells and tissues [65]. Polymeric nanoparticles have been extensively explored as means for drug solubilization, stabilization and targeting [66]. Polymeric nanoparticles possess several unique characteristics for antimicrobial drug delivery. Firstly, polymeric nanoparticles are structurally stable and can be synthesized with a sharper size distribution. Secondly, particle properties such as size, zeta potentials, and drug release profiles can be accurately tuned by selecting different polymer lengths, surfactants, and organic solvents during the synthesis. Thirdly, the surface of poly‐ meric nanoparticles typically contains functional groups that can be chemically changed with either drug moieties or targeting ligands [67]. For targeted antimicrobial delivery, polymeric

minimizing side effects [33].

160 Application of Nanotechnology in Drug Delivery

**7.1. Polymeric nanoparticles**

**7. Types of drug carriers in medicine**

A hydrogel is a network of hydrophilic polymers that can swell in water and hold a large amount of water while maintaining the structure [69]. Drugs can be loaded into the polymer matrix of these materials and controlled release is dependent on the diffusion coefficient of the drug across the hydrogel network [70]. Amongst the several types of drug delivery systems that have been developed in order to improve effectiveness and biocompatibility, hydrogels are extremely promising. Hydrogels are biocompatible hydrophilic networks that can be constructed from both synthetic and natural materials [71]. In an overall view, hydrogels can be classified based on a variety of characteristics, containing the nature of side groups (neutral or ionic), mechanical and structural features (affine or phantom), method of preparation (homo-or co-polymer), physical structure (amorphous, semicrystalline, hydrogen bonded, supermolecular, and hydrocollodial), and responsiveness to physiologic environment stimuli (pH, ionic strength, temperature, electromagnetic radiation, etc.) [72-75]. Classically, hydro‐ gels have been used to deliver hydrophilic, small-molecule drugs which have high solubilities in both the hydrophilic hydrogel matrix and the aqueous solvent swelling the hydrogel [76]. Hydrogel-based hydrophobic drug delivery is in many respects a more difficult problem given the innate incongruity of the hydrophilic hydrogel network and the hydrophobic drug. A variety of strategies for introducing hydrophobic domains directly into otherwise hydrophilic hydrogel networks have permitted significant improvements in the loading of hydrophobic drugs [76]. Hydrogel/glass composite (Nitric oxide-releasing nanoparticles) NO NPs have also been shown to have a high degree of effectiveness against (Methicillin-resistant *Staphylococcus aureus*) MRSA infection in several different mouse models. In one mouse study by Martinez et al., administration of topical hydrogel/glass composite NO NPs into skin wounds infected with MRSA reduced bacterial burden significantly compared to controls [77]. Despite these many advantageous properties, hydrogels also have several limitations. The low elastic force of many hydrogels limits their use in load-bearing applications and can result in the precocious decomposition or flow away of the hydrogel from a targeted local site. This limitation may not be important in many typical drug delivery applications (e.g. subcutaneous injection) [78].

#### **7.3. Metal nanoparticles**

Metal-based nanoparticles of different shapes, sizes (between 10 to 100 nm) have also been investigated as diagnostic and drug delivery systems. Most common metallic nanoparticles contain gold, nickel, silver, iron oxide, zinc oxide, gadolinium, and titanium dioxide particles [79]. Metal nanoparticles, which have a high specific surface area and a high fraction of surface atoms, have been studied extensively because of their unique physicochemical characteristics including catalytic activity, optical properties, electronic properties, antimicrobial activity, and magnetic properties [80-82]. Even though metallic nanoparticles are biocompatible and immobile carriers, a significant fraction of metal particles can be retained and accumulated in the body after drug administration, probably causing toxicity. Consequently, the use of metallic nanoparticles for drug delivery is a concern [83].

the most effective against bacteria, viruses, and other eukaryotic microorganisms [98, 99]. Antibacterial properties inhibit the reproduction of bacteria, which is a microbe. The silver nanoparticles can "inactivate proteins, blocking respiration and electron transfer, and subse‐ quently inactivating the bacteria" [100]. The antibacterial properties of the silver nanoparticles depend on the size of the particles; the smaller the particles the better the effect. The particle size is a major factor because the smaller the particle the greater the surface area, which allows for greater interaction with the bacteria [100]. It has been reported that combined use of silver nanoparticles with antibiotics, such as penicillin G, amoxicillin, erythromycin, and vancomy‐ cin, resulted in enhanced and synergistic antimicrobial effects against Gram-positive and Gram-negative bacteria (e.g., *E. coli* and *S. aureus*) [80, 101, 102]. Although beneficial as antimicrobial agents, silver nanoparticles have adverse effects on cells such as the production of reactive oxygen species which are toxic to both bacteria and eukaryotic cells [103, 104]. In contrast, the cytotoxicity of gold nanoparticles is quite low, and they have been used for

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Magnetic nanoparticles engineered as drug delivery devices retain the ability to track their movement through the body. This is significant because it allows clinicians to monitor the effectivity of injected therapeutics to reach their target sites [107]. Iron oxide nanoparticles (IONPs) are magnetic Fe3O4 or Fe2O3 nanocrystals which can interact with external magnetic fields, offering different opportunities in nanomedicine, e.g., as contrast agents in MRI, for magnetic hyperthermal therapies, or as magnetically triggerable drug delivery systems [108]. There are some studies on evaluating the toxicity of magnetite nanoparticles on eukaryote cells, which their results showed negligible toxicity in eukaryote cells of the modified mag‐ netite nanoparticles with different surfactants such as glycine or oleic acid. But the toxicity of magnetite nanoparticles on bacteria cells has not been reported [109]. However, in most of the cases where magnetic nanocarriers have been used, difficulties in achieving these objectives appeared. In turn, magnetic force may not be strong enough to overcome the force of blood flow and to accumulate magnetic drugs only at target site [110]. Therefore, designing magnetic drug delivery systems requires taking into consideration many factors, e.g., magnetic prop‐ erties and size of particles, strength of magnetic field, drug loading capacity, the place of accessibility of target tissue, or the rate of blood flow [111]. The vancomycin functionalized magnetic nanoparticles for pathogen detection have been investigated by Gu et al. [112]. Vancomycin can be attached to the magnetic nanoparticles surface by activating the–COOH group of vancomycin followed by reaction with the amine groups on the surface of the iron oxide nanoparticles. The vancomycin conjugated iron oxide nanoparticles were utilized as probes to selectively entrap *S. saprophyticus* (a pathogen that usually infects the urinary tract of young women) and *S. aureus* bacteria from urine specimen using a magnetic field [1, 112]. It has been reported that the various nanoparticles, Al2O3, Fe3O4, CeO2, ZrO2 and MgO were subjected to evaluate its antibacterial potential against ophthalmic pathogens such as *Pseudo‐ monas aeruginosa, Acinetobacter* sp., *Klebsiella pneumoniae, E. coli, Streptococcus viridans* and *Streptococcus pyogenes*. Among the nanoparticles, Fe3O4 showed maximum activity against

medical imaging and have served as scaffolds for drug delivery [105, 106].

*7.3.3. Magnetic nanoparticles*

#### *7.3.1. Gold nanoparticles*

Gold nanoparticles (GNPs) have found many applications in many fields such as cancer diagnosis and therapy, drug and gene delivery, DNA and ptotein determination, etc. Due to their unique properties of small size, large surface area to volume ratio, high reactivity to the living cells, stability over high temperatures and translocation into the cells [84]. GNPs are suitable for the delivery of drugs to cellular destinations due to their ease of synthesis, functionalization and biocompatibility. GNPs functionalized with targeted specific biomole‐ cules can effectively destroy cancer cells or bacteria [85]. The efficacy of GNPs conjugated to several antibiotics has also been the subject of some studies by Grace and Saha et al. They discovered that GNPs conjugates were more efficient in inhibiting the growth of Gram-positive and Gram-negative bacteria in comparison with the same dosage of antibiotics utilized alone. Their results suggest that GNPs can act as an effective drug carrier in a drug delivery system [86, 87]. Conjugates of gold nanoparticles with antibiotics and antibodies also have been used for selective photothermal killing of protozoa and bacteria [88]. Gu et al. synthesized stable gold nanoparticles covered with vancomycin and showed significant enhancement of anti‐ bacterial activity, in comparison with the activity of the free antibiotic [89]. In another report, Selvaraj et al. utilized the anticancer compound 5-fluorouracil bound to GNPs and found that the resulting conjugate was significantly more effective against a range of bacterial and fungal organisms in comparison with alone [90]. Recently, it has been reported that the gentamicin conjugated with gold nanospheres was significantly more effective against *S. aureus* in comparsion with free gentamicin [91]. Each GNP surrounded by a number of drug moieties acts as a single group against the microbial organisms [92]. The greater antibacterial effect of the GNPs conjugates has been ascribed to their ability to bind to and/or penetrate the cell wall and, in doing so they are able to deliver a large number of antibiotic molecules into a highly localized volume [93].

#### *7.3.2. Silver nanoparticles*

Silver nanoparticles of size smaller than 100 nm contain about 10000–15000 silver atoms [94, 95]. They are prepared by engineering the metallic silver into ultrafine particles by numerous physical methods, which include spark discharging, electrochemical reduction, solution irradiation and cryochemical synthesis [96]. The most widely used and known application of silver nanoparticles is in the medical sciences. These include topical ointments and creams containing silver to prevent infection of burns and open wounds [97]. Among the many different types of metallic and metal oxide NPs, silver nanoparticles have demonstrated to be

the most effective against bacteria, viruses, and other eukaryotic microorganisms [98, 99]. Antibacterial properties inhibit the reproduction of bacteria, which is a microbe. The silver nanoparticles can "inactivate proteins, blocking respiration and electron transfer, and subse‐ quently inactivating the bacteria" [100]. The antibacterial properties of the silver nanoparticles depend on the size of the particles; the smaller the particles the better the effect. The particle size is a major factor because the smaller the particle the greater the surface area, which allows for greater interaction with the bacteria [100]. It has been reported that combined use of silver nanoparticles with antibiotics, such as penicillin G, amoxicillin, erythromycin, and vancomy‐ cin, resulted in enhanced and synergistic antimicrobial effects against Gram-positive and Gram-negative bacteria (e.g., *E. coli* and *S. aureus*) [80, 101, 102]. Although beneficial as antimicrobial agents, silver nanoparticles have adverse effects on cells such as the production of reactive oxygen species which are toxic to both bacteria and eukaryotic cells [103, 104]. In contrast, the cytotoxicity of gold nanoparticles is quite low, and they have been used for medical imaging and have served as scaffolds for drug delivery [105, 106].

#### *7.3.3. Magnetic nanoparticles*

[79]. Metal nanoparticles, which have a high specific surface area and a high fraction of surface atoms, have been studied extensively because of their unique physicochemical characteristics including catalytic activity, optical properties, electronic properties, antimicrobial activity, and magnetic properties [80-82]. Even though metallic nanoparticles are biocompatible and immobile carriers, a significant fraction of metal particles can be retained and accumulated in the body after drug administration, probably causing toxicity. Consequently, the use of

Gold nanoparticles (GNPs) have found many applications in many fields such as cancer diagnosis and therapy, drug and gene delivery, DNA and ptotein determination, etc. Due to their unique properties of small size, large surface area to volume ratio, high reactivity to the living cells, stability over high temperatures and translocation into the cells [84]. GNPs are suitable for the delivery of drugs to cellular destinations due to their ease of synthesis, functionalization and biocompatibility. GNPs functionalized with targeted specific biomole‐ cules can effectively destroy cancer cells or bacteria [85]. The efficacy of GNPs conjugated to several antibiotics has also been the subject of some studies by Grace and Saha et al. They discovered that GNPs conjugates were more efficient in inhibiting the growth of Gram-positive and Gram-negative bacteria in comparison with the same dosage of antibiotics utilized alone. Their results suggest that GNPs can act as an effective drug carrier in a drug delivery system [86, 87]. Conjugates of gold nanoparticles with antibiotics and antibodies also have been used for selective photothermal killing of protozoa and bacteria [88]. Gu et al. synthesized stable gold nanoparticles covered with vancomycin and showed significant enhancement of anti‐ bacterial activity, in comparison with the activity of the free antibiotic [89]. In another report, Selvaraj et al. utilized the anticancer compound 5-fluorouracil bound to GNPs and found that the resulting conjugate was significantly more effective against a range of bacterial and fungal organisms in comparison with alone [90]. Recently, it has been reported that the gentamicin conjugated with gold nanospheres was significantly more effective against *S. aureus* in comparsion with free gentamicin [91]. Each GNP surrounded by a number of drug moieties acts as a single group against the microbial organisms [92]. The greater antibacterial effect of the GNPs conjugates has been ascribed to their ability to bind to and/or penetrate the cell wall and, in doing so they are able to deliver a large number of antibiotic molecules into a highly

Silver nanoparticles of size smaller than 100 nm contain about 10000–15000 silver atoms [94, 95]. They are prepared by engineering the metallic silver into ultrafine particles by numerous physical methods, which include spark discharging, electrochemical reduction, solution irradiation and cryochemical synthesis [96]. The most widely used and known application of silver nanoparticles is in the medical sciences. These include topical ointments and creams containing silver to prevent infection of burns and open wounds [97]. Among the many different types of metallic and metal oxide NPs, silver nanoparticles have demonstrated to be

metallic nanoparticles for drug delivery is a concern [83].

*7.3.1. Gold nanoparticles*

162 Application of Nanotechnology in Drug Delivery

localized volume [93].

*7.3.2. Silver nanoparticles*

Magnetic nanoparticles engineered as drug delivery devices retain the ability to track their movement through the body. This is significant because it allows clinicians to monitor the effectivity of injected therapeutics to reach their target sites [107]. Iron oxide nanoparticles (IONPs) are magnetic Fe3O4 or Fe2O3 nanocrystals which can interact with external magnetic fields, offering different opportunities in nanomedicine, e.g., as contrast agents in MRI, for magnetic hyperthermal therapies, or as magnetically triggerable drug delivery systems [108]. There are some studies on evaluating the toxicity of magnetite nanoparticles on eukaryote cells, which their results showed negligible toxicity in eukaryote cells of the modified mag‐ netite nanoparticles with different surfactants such as glycine or oleic acid. But the toxicity of magnetite nanoparticles on bacteria cells has not been reported [109]. However, in most of the cases where magnetic nanocarriers have been used, difficulties in achieving these objectives appeared. In turn, magnetic force may not be strong enough to overcome the force of blood flow and to accumulate magnetic drugs only at target site [110]. Therefore, designing magnetic drug delivery systems requires taking into consideration many factors, e.g., magnetic prop‐ erties and size of particles, strength of magnetic field, drug loading capacity, the place of accessibility of target tissue, or the rate of blood flow [111]. The vancomycin functionalized magnetic nanoparticles for pathogen detection have been investigated by Gu et al. [112]. Vancomycin can be attached to the magnetic nanoparticles surface by activating the–COOH group of vancomycin followed by reaction with the amine groups on the surface of the iron oxide nanoparticles. The vancomycin conjugated iron oxide nanoparticles were utilized as probes to selectively entrap *S. saprophyticus* (a pathogen that usually infects the urinary tract of young women) and *S. aureus* bacteria from urine specimen using a magnetic field [1, 112]. It has been reported that the various nanoparticles, Al2O3, Fe3O4, CeO2, ZrO2 and MgO were subjected to evaluate its antibacterial potential against ophthalmic pathogens such as *Pseudo‐ monas aeruginosa, Acinetobacter* sp., *Klebsiella pneumoniae, E. coli, Streptococcus viridans* and *Streptococcus pyogenes*. Among the nanoparticles, Fe3O4 showed maximum activity against *Pseudomonas aeruginosa.* The reactive oxygen species (ROS) generated by Fe3O4 nanoparticles could kill bacteria without harming nonbacterial cells [113].

slower extravazation of polymeric carrier systems than that of low molecular weight drugs. This results from a difference in extravazation mechanisms between polymeric carrier systems

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Liposomes are small spherical vesicles in which one or more aqueous parts are completely surrounded by molecules that have hydrophilic and hydrophobic functionality. Liposomes change with composition, size, surface charge and method of preparation. They can be single or in multiple bilayers. Those including one bilayer membrane are called small unilamellar vesicles or large unilamellar vesicles based on their sizes [127]. Nanoparticulate DDS, such as liposomes, are mostly used to enhance the efficacy of drug and DNA delivery and targeting [128, 129]. Liposomes are also the most broadly used antimicrobial drug delivery vehicles because their lipid bilayer structure imitators the cell membrane and can readily fuse with infectious microbes [30]. One of the disadvantages of liposomal antibiotics is the short shelflives of lipid vesicles, which limits drug stability. Short shelflives can be conditioned by both physical and chemical processes [130]. There are many advantages of liposomes as antibiotic carriers: improved pharmacokinetics and biodistribution; decreased toxicity; enhanced activity against intracellular pathogens; target selectivity; enhanced activity against extracel‐ lular pathogens, in particular to overcome bacterial drug resistance [131]. The ability of liposomes to alter drug distribution depends mostly on their size and surface properties [132]. Thus, liposomal encapsulation of antibiotics helps to increase their therapeutic index with mode of action related to increasing the drug concentration at the site of infection and/or reducing its toxicity [133]. For instance, encapsulation of vancomycin and teicoplanin in liposomes resulted in significantly improved elimination of intracellular methicillin resistant *S*. *aureus* (MRSA) infection [35]. Netilmicin liposomes showed an increase in pharmacological activity in a peritonitis model of mice infected with *E*. *coli*, in terms of survival both prophy‐ lactically and therapeutically [134]. Recently, Deol and Khuller produced lung-specific liposomes made of phosphatidylcholine, cholesterol, dicetylphosphate, O-steroyl amylopectin and monosialogangliosides/distearylphosphatidylethanolamine-poly (ethylene glycol) 2000

for the targeted delivery of anti-Tuberculosis (TB) drugs to the lung [135].

Solid lipid nanoparticles (SLN) were developed at the beginning of 1990s as an alterna‐ tive carrier system to emulsions, liposomes and polymeric nanoparticles as a colloidal carrier system for controlled drug delivery [20]. SLNs are sub-micron colloidal carriers, ranging from 50 nm to 1 μm, that are composed of physiological lipid dispersed in water or in aqueous surfactant solution [136]. In the last decade SLNs have gained considerable interest as novel particulate drug delivery systems. SLNs are suitable for the incorporation of lipophilic and hydrophilic drugs within the lipid matrix in considerable amounts [137]. SLN consist of a solid lipid matrix at room and body temperature, where the drug is normally incorporated in the submicron size range (below 1 *µ*m) [35]. Some advantages of SLNs are

**7.7. Solid lipid nanoparticles (SLN)**

and low molecular weight drugs [126].

**7.6. Liposomes**

#### **7.4. Silica nanoparticles**

Silica materials are suitable for several important biological applications, such as drug delivery, imaging, oxygen carrier or controlled release [114]. Silica materials have been proved to be efficient carriers for the local release of antibiotics, which could be of interest in the context of biofilm associated infections, which are a real challenge for the modern medicine [115]. Moreover, mesoporous silica has been found to be relatively "non-toxic" and biocompatible, however of course depending on dose and administration route [116]. Nanoporous silica materials possess large pore volumes and high surface areas, allowing the absorption of large amounts of drugs, thus providing sufficient concentrations for local treatment. The surface of silica materials is reactive due to the presence of silanol groups. This allows for facile modifi‐ cation by silanization reactions and thus opens possibilities for enhancing the drug loading and for controlling the drug release [117]. Till present there are only few reports concerning the application of silica materials, crystalline or amorphous, in the antimicrobial therapy [115]. Zhang et al. suggested a highly-sensitive fluoroimmunoassay for the determination of *staphylococcal* enterotoxin C1 (SEC1). This method utilizes anti-SEC1 coated NPs for detection which is possible in food samples and enables fluorescence microscopy imaging for the determination of SEC1 [118]. Recently, Grumezescu et al. reported that silica nanostructures have significantly improved the anti-*staphylococcal* activity of bacitracin and kanamycin sulfate, as revealed by the drastic decrease of the minimal inhibitory activity of the respective antibiotics loaded in the SiO2 nanopowder. These results, correlated with the high biocom‐ patibility of the porous silica structure recommend it as an efficient vehicle for the local delivery of antibiotics in lower active doses, reducing thus their cytotoxicity and side effects [119].

#### **7.5. Micelles**

Micelles are submicroscopic aggregates of surfactant molecules assembly of amphiphillic block copolymers or polymer-lipid conjugates or other surface-active molecules that selfassemble in aqueous media to form structures with a hydrophobic core [120, 121]. The ability to functionalise the micelles as well as tailor the disintegration behaviour by varying the copolymer composition are beneficial parameters in making them drug carriers of choice. Their small size (1-50 nm) makes them ideal for intravenous delivery. In addition they are also more stable, when compared to liposomes due to be ability to design them to be chemically stable and biocompatible [122]. One specific feature of micelles is that the amount of drug released can be controlled by an external stimulus like pH, temperature, ultrasound or certain enzymes [123]. Other unique properties of polymeric micelles are that they are easily altered with small functional groups that enhance their targeting potential [124]. Generally, polymeric surfactants are known to be less toxic than low-molecular-weight surfactants, such as sodium dodecyl sulfate. Furthermore, in theory, polymeric micelles are considered very safe in relation to chronic toxicity [125]. The disadvantage for the polymeric micelle systems is the immature technology for drug incorporation in a physical manner. The another disadvantage is much slower extravazation of polymeric carrier systems than that of low molecular weight drugs. This results from a difference in extravazation mechanisms between polymeric carrier systems and low molecular weight drugs [126].

#### **7.6. Liposomes**

*Pseudomonas aeruginosa.* The reactive oxygen species (ROS) generated by Fe3O4 nanoparticles

Silica materials are suitable for several important biological applications, such as drug delivery, imaging, oxygen carrier or controlled release [114]. Silica materials have been proved to be efficient carriers for the local release of antibiotics, which could be of interest in the context of biofilm associated infections, which are a real challenge for the modern medicine [115]. Moreover, mesoporous silica has been found to be relatively "non-toxic" and biocompatible, however of course depending on dose and administration route [116]. Nanoporous silica materials possess large pore volumes and high surface areas, allowing the absorption of large amounts of drugs, thus providing sufficient concentrations for local treatment. The surface of silica materials is reactive due to the presence of silanol groups. This allows for facile modifi‐ cation by silanization reactions and thus opens possibilities for enhancing the drug loading and for controlling the drug release [117]. Till present there are only few reports concerning the application of silica materials, crystalline or amorphous, in the antimicrobial therapy [115]. Zhang et al. suggested a highly-sensitive fluoroimmunoassay for the determination of *staphylococcal* enterotoxin C1 (SEC1). This method utilizes anti-SEC1 coated NPs for detection which is possible in food samples and enables fluorescence microscopy imaging for the determination of SEC1 [118]. Recently, Grumezescu et al. reported that silica nanostructures have significantly improved the anti-*staphylococcal* activity of bacitracin and kanamycin sulfate, as revealed by the drastic decrease of the minimal inhibitory activity of the respective antibiotics loaded in the SiO2 nanopowder. These results, correlated with the high biocom‐ patibility of the porous silica structure recommend it as an efficient vehicle for the local delivery of antibiotics in lower active doses, reducing thus their cytotoxicity and side effects [119].

Micelles are submicroscopic aggregates of surfactant molecules assembly of amphiphillic block copolymers or polymer-lipid conjugates or other surface-active molecules that selfassemble in aqueous media to form structures with a hydrophobic core [120, 121]. The ability to functionalise the micelles as well as tailor the disintegration behaviour by varying the copolymer composition are beneficial parameters in making them drug carriers of choice. Their small size (1-50 nm) makes them ideal for intravenous delivery. In addition they are also more stable, when compared to liposomes due to be ability to design them to be chemically stable and biocompatible [122]. One specific feature of micelles is that the amount of drug released can be controlled by an external stimulus like pH, temperature, ultrasound or certain enzymes [123]. Other unique properties of polymeric micelles are that they are easily altered with small functional groups that enhance their targeting potential [124]. Generally, polymeric surfactants are known to be less toxic than low-molecular-weight surfactants, such as sodium dodecyl sulfate. Furthermore, in theory, polymeric micelles are considered very safe in relation to chronic toxicity [125]. The disadvantage for the polymeric micelle systems is the immature technology for drug incorporation in a physical manner. The another disadvantage is much

could kill bacteria without harming nonbacterial cells [113].

**7.4. Silica nanoparticles**

164 Application of Nanotechnology in Drug Delivery

**7.5. Micelles**

Liposomes are small spherical vesicles in which one or more aqueous parts are completely surrounded by molecules that have hydrophilic and hydrophobic functionality. Liposomes change with composition, size, surface charge and method of preparation. They can be single or in multiple bilayers. Those including one bilayer membrane are called small unilamellar vesicles or large unilamellar vesicles based on their sizes [127]. Nanoparticulate DDS, such as liposomes, are mostly used to enhance the efficacy of drug and DNA delivery and targeting [128, 129]. Liposomes are also the most broadly used antimicrobial drug delivery vehicles because their lipid bilayer structure imitators the cell membrane and can readily fuse with infectious microbes [30]. One of the disadvantages of liposomal antibiotics is the short shelflives of lipid vesicles, which limits drug stability. Short shelflives can be conditioned by both physical and chemical processes [130]. There are many advantages of liposomes as antibiotic carriers: improved pharmacokinetics and biodistribution; decreased toxicity; enhanced activity against intracellular pathogens; target selectivity; enhanced activity against extracel‐ lular pathogens, in particular to overcome bacterial drug resistance [131]. The ability of liposomes to alter drug distribution depends mostly on their size and surface properties [132]. Thus, liposomal encapsulation of antibiotics helps to increase their therapeutic index with mode of action related to increasing the drug concentration at the site of infection and/or reducing its toxicity [133]. For instance, encapsulation of vancomycin and teicoplanin in liposomes resulted in significantly improved elimination of intracellular methicillin resistant *S*. *aureus* (MRSA) infection [35]. Netilmicin liposomes showed an increase in pharmacological activity in a peritonitis model of mice infected with *E*. *coli*, in terms of survival both prophy‐ lactically and therapeutically [134]. Recently, Deol and Khuller produced lung-specific liposomes made of phosphatidylcholine, cholesterol, dicetylphosphate, O-steroyl amylopectin and monosialogangliosides/distearylphosphatidylethanolamine-poly (ethylene glycol) 2000 for the targeted delivery of anti-Tuberculosis (TB) drugs to the lung [135].

#### **7.7. Solid lipid nanoparticles (SLN)**

Solid lipid nanoparticles (SLN) were developed at the beginning of 1990s as an alterna‐ tive carrier system to emulsions, liposomes and polymeric nanoparticles as a colloidal carrier system for controlled drug delivery [20]. SLNs are sub-micron colloidal carriers, ranging from 50 nm to 1 μm, that are composed of physiological lipid dispersed in water or in aqueous surfactant solution [136]. In the last decade SLNs have gained considerable interest as novel particulate drug delivery systems. SLNs are suitable for the incorporation of lipophilic and hydrophilic drugs within the lipid matrix in considerable amounts [137]. SLN consist of a solid lipid matrix at room and body temperature, where the drug is normally incorporated in the submicron size range (below 1 *µ*m) [35]. Some advantages of SLNs are possibility of controlling drug release and drug targeting, increased drug stability, high drug payload, possibility of the incorporation of lipophilic and hydrophilic drugs, lack of biotoxicity of the carrier, no problems with respect to large-scale production, sterilization possibility, and good tolerability [138]. Common disadvantages of SLN are their particle growing, their unpredictable gelation tendency, their unexpected dynamics of polymor‐ phic transitions and their inherent low incorporation rate due to the crystalline structure of the solid lipid [139]. SLNs are considered good drug carriers to obtain sustained release of antibiotics [140]. SLNs can act as promising carriers for sustained ciprofloxacin release in infections or to enhance the bioavailability of tobramycin from antibiotic-loaded SLN in the aqueous humor for topical ocular delivery [141, 142]. Nimje et al. (2009) reported the selective delivery of rifabutin, another antituberculosis drug, to alveolar tissues, using drugloaded solid lipid nanoparticles, increasing the therapeutic margin of safety and reducing side effects [143]. Another prominent example of SLNs-based drug delivery is pulmonary delivery of antimicrobials to treat tuberculosis, a serious lung infection caused by *Mycobac‐ terium tuberculosis*. In some severe cases, tuberculosis infection spreads from the lungs and affects the lymphatic systems. SLNs can facilitate the delivery of anti-tuberculosis drugs such as rifampin, isoniazidand pyrazinamide to the lungs as well as to the lymphatic systems [144]. Even though the development history of SLN-based antimicrobial drug delivery systems is relatively shorter than other nanoparticle systems such as liposomes and polymeric nanoparticles, SLNs have shown great therapeutic potentials [145].

Gram positive and Gram negative bacteria [154]. Additionally, the quinazolin–fullerene conjugate 18 was reported to have an inhibitory potential of 98.83% at a minimal inhibitory

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First discovered in the early 1980's by Tomalia and co-workers, such hyperbranched molecules were called dendrimers [156]. Dendrimers are globular repeatedly branched macromolecules that exhibit controlled patterns of branching with multiple arms extending from a central core [157]. The well defined structure, monodispersity of size, surface functionalization capability, and stability are properties of dendrimers that make them attractive drug carrier candidates [20]. Asymmetric dendrimers are synthesized by coupling dendrons of different generations (G1-G4) to a linear core, which yields a branched dendrimer with a nonuniform orthogonal architecture. This asymmetry allows for tunable structures and molecular weights, with precise control over the number of functional groups available on each dendron for attachment of drugs, imaging agents, and other therapeutic moieties [158]. Dendrimers also possess many unique properties that make them a good nanoparticle platform for antimicrobial drug delivery. They are highly arranged and regularly branched globular macromolecules, with a core, layers of branched repeat units emerging from the core and functional end groups on the outer layer of repeat units [159]. Dendrimer biocides may contain quaternary ammonium salts as functional end groups displaying greater antimicrobial activity against bacteria than small drug molecules, due to a high density of active antimicrobials on the dendrimer surfaces [160]. Dendrimers can be made from a wide variety of biocompatible materials, the most frequently used are polyamidoamine (PAMAM), polyethylene oxide (PEO), polypropylene imine (PPI), polyethyleneimine (PEI), polyethylene glycol (PEG) etc [161]. PAMAM dendrimers are dendritic polymers characterized by regular branching and radial symmetry. PAMAM dendrimers have illustrated useful drug delivery and antimicrobial applications with aminoterminated dendrimers showing high antibacterial efficacy [162]. It is well known that PAMAM dendrimers with primary amine surface functional groups may enter the cellular membrane. Sulfomethoxazole (a sulfonamide derivative poorly soluble and thus presenting low bioavailability) was administered with PAMAM dendrimers *in vitro* [163]. Sulfamethox‐ azole (SMZ)-encapsulating PAMAM dendrimers led to sustained release of the drug *in vitro* and 4–8 folds increased antibacterial activity against *E. coli,* compared to free SMZ [163].

Zeolites are solid hydrated crystalline materials with frame-works comprising silicon, aluminum and oxygen and featuring nano-channels and cages of regular dimensions [164]. Silica is a neutral regular tetrahedronin in which positive charge of silicon ion is balanced by oxygen [165]. The capacity of cation exchange depends on the ratio of silica/alumina in the structure. Generally, zeolits with a low silica/alumina (Si/Al) ratio have higher ion exchange capacity. According Si/Al ratio, there are several types of natural and synthetic zeolites including zeolite-β, zeolite A, zeolite X and zeolite Y, which are the most common commercial adsorbents [165]. Zeolites are minerals with selective pores that can be used to sieve molecules

concentration of 1.562 μg/mL when treating *M. tuberculosis* [155].

**7.9. Dendrimers**

**7.10. Zeolites**

#### **7.8. Fullerenes**

Fullerenes are a new form of carbon, other forms being diamond, graphite, and coal. They can take three forms of a hollow sphere, ellipsoid, or tube. Their small size, spherical shape, and hollow interior all provide therapeutic opportunities [146]. The most abundant form of fullerenes is buckminsterfullerene (C60) with 60 carbon atoms arranged in a spherical structure [147]. The shape of the molecule, recognized as truncated icosahedron, resembles that of a football ball, containing 12 pentagons and 20 hexagons, in which every carbon atom forms bond to three other neighbor atoms through sp2 hybridization [148]. Friedman et al and Schinazi et al distinguished that the hydrophobic cleft of the human immunodeficiency virus (HIV)-1 protease can seamlessly host a C60 molecule [149]. This discovery was the first piece of evidence that fullerenes could have pharmaceutical significance through interactions with biological targets, highlighting the great potential of fullerenes in medicinal applications. Since fullerenes possess unique geometrical shapes, as well as novel photophysical properties, in addition to being efficient radical scavengers, a wide variety of biological applications have been considered [150-152]. Some studies asserted that C60 could be also utilized for the photodynamic inactivation of bacteria, as persuasively demonstrated in studies examining the effects of water-soluble and nanoparticulate C60 on various bacterial strains [153]. The effects were significantly more pronounced in Gram positive *(Staphylococcus* spp., *Streptococcus* spp.) than in Gram negative bacteria (*Klebsiella Pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Streptococcus pyogenes*), indicating that the bactericidal action was dependent on the fullerene insertion into the microbial cellwall, the structure of which differs between Gram positive and Gram negative bacteria [154]. Additionally, the quinazolin–fullerene conjugate 18 was reported to have an inhibitory potential of 98.83% at a minimal inhibitory concentration of 1.562 μg/mL when treating *M. tuberculosis* [155].

#### **7.9. Dendrimers**

possibility of controlling drug release and drug targeting, increased drug stability, high drug payload, possibility of the incorporation of lipophilic and hydrophilic drugs, lack of biotoxicity of the carrier, no problems with respect to large-scale production, sterilization possibility, and good tolerability [138]. Common disadvantages of SLN are their particle growing, their unpredictable gelation tendency, their unexpected dynamics of polymor‐ phic transitions and their inherent low incorporation rate due to the crystalline structure of the solid lipid [139]. SLNs are considered good drug carriers to obtain sustained release of antibiotics [140]. SLNs can act as promising carriers for sustained ciprofloxacin release in infections or to enhance the bioavailability of tobramycin from antibiotic-loaded SLN in the aqueous humor for topical ocular delivery [141, 142]. Nimje et al. (2009) reported the selective delivery of rifabutin, another antituberculosis drug, to alveolar tissues, using drugloaded solid lipid nanoparticles, increasing the therapeutic margin of safety and reducing side effects [143]. Another prominent example of SLNs-based drug delivery is pulmonary delivery of antimicrobials to treat tuberculosis, a serious lung infection caused by *Mycobac‐ terium tuberculosis*. In some severe cases, tuberculosis infection spreads from the lungs and affects the lymphatic systems. SLNs can facilitate the delivery of anti-tuberculosis drugs such as rifampin, isoniazidand pyrazinamide to the lungs as well as to the lymphatic systems [144]. Even though the development history of SLN-based antimicrobial drug delivery systems is relatively shorter than other nanoparticle systems such as liposomes

and polymeric nanoparticles, SLNs have shown great therapeutic potentials [145].

Fullerenes are a new form of carbon, other forms being diamond, graphite, and coal. They can take three forms of a hollow sphere, ellipsoid, or tube. Their small size, spherical shape, and hollow interior all provide therapeutic opportunities [146]. The most abundant form of fullerenes is buckminsterfullerene (C60) with 60 carbon atoms arranged in a spherical structure [147]. The shape of the molecule, recognized as truncated icosahedron, resembles that of a football ball, containing 12 pentagons and 20 hexagons, in which every carbon atom forms

Schinazi et al distinguished that the hydrophobic cleft of the human immunodeficiency virus (HIV)-1 protease can seamlessly host a C60 molecule [149]. This discovery was the first piece of evidence that fullerenes could have pharmaceutical significance through interactions with biological targets, highlighting the great potential of fullerenes in medicinal applications. Since fullerenes possess unique geometrical shapes, as well as novel photophysical properties, in addition to being efficient radical scavengers, a wide variety of biological applications have been considered [150-152]. Some studies asserted that C60 could be also utilized for the photodynamic inactivation of bacteria, as persuasively demonstrated in studies examining the effects of water-soluble and nanoparticulate C60 on various bacterial strains [153]. The effects were significantly more pronounced in Gram positive *(Staphylococcus* spp., *Streptococcus* spp.) than in Gram negative bacteria (*Klebsiella Pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Streptococcus pyogenes*), indicating that the bactericidal action was dependent on the fullerene insertion into the microbial cellwall, the structure of which differs between

hybridization [148]. Friedman et al and

**7.8. Fullerenes**

166 Application of Nanotechnology in Drug Delivery

bond to three other neighbor atoms through sp2

First discovered in the early 1980's by Tomalia and co-workers, such hyperbranched molecules were called dendrimers [156]. Dendrimers are globular repeatedly branched macromolecules that exhibit controlled patterns of branching with multiple arms extending from a central core [157]. The well defined structure, monodispersity of size, surface functionalization capability, and stability are properties of dendrimers that make them attractive drug carrier candidates [20]. Asymmetric dendrimers are synthesized by coupling dendrons of different generations (G1-G4) to a linear core, which yields a branched dendrimer with a nonuniform orthogonal architecture. This asymmetry allows for tunable structures and molecular weights, with precise control over the number of functional groups available on each dendron for attachment of drugs, imaging agents, and other therapeutic moieties [158]. Dendrimers also possess many unique properties that make them a good nanoparticle platform for antimicrobial drug delivery. They are highly arranged and regularly branched globular macromolecules, with a core, layers of branched repeat units emerging from the core and functional end groups on the outer layer of repeat units [159]. Dendrimer biocides may contain quaternary ammonium salts as functional end groups displaying greater antimicrobial activity against bacteria than small drug molecules, due to a high density of active antimicrobials on the dendrimer surfaces [160]. Dendrimers can be made from a wide variety of biocompatible materials, the most frequently used are polyamidoamine (PAMAM), polyethylene oxide (PEO), polypropylene imine (PPI), polyethyleneimine (PEI), polyethylene glycol (PEG) etc [161]. PAMAM dendrimers are dendritic polymers characterized by regular branching and radial symmetry. PAMAM dendrimers have illustrated useful drug delivery and antimicrobial applications with aminoterminated dendrimers showing high antibacterial efficacy [162]. It is well known that PAMAM dendrimers with primary amine surface functional groups may enter the cellular membrane. Sulfomethoxazole (a sulfonamide derivative poorly soluble and thus presenting low bioavailability) was administered with PAMAM dendrimers *in vitro* [163]. Sulfamethox‐ azole (SMZ)-encapsulating PAMAM dendrimers led to sustained release of the drug *in vitro* and 4–8 folds increased antibacterial activity against *E. coli,* compared to free SMZ [163].

#### **7.10. Zeolites**

Zeolites are solid hydrated crystalline materials with frame-works comprising silicon, aluminum and oxygen and featuring nano-channels and cages of regular dimensions [164]. Silica is a neutral regular tetrahedronin in which positive charge of silicon ion is balanced by oxygen [165]. The capacity of cation exchange depends on the ratio of silica/alumina in the structure. Generally, zeolits with a low silica/alumina (Si/Al) ratio have higher ion exchange capacity. According Si/Al ratio, there are several types of natural and synthetic zeolites including zeolite-β, zeolite A, zeolite X and zeolite Y, which are the most common commercial adsorbents [165]. Zeolites are minerals with selective pores that can be used to sieve molecules having certain dimensions [166]. Several recent studies showed that the potential of zeolites in medical applications is due to their structural properties and stability in biological envi‐ ronments [167]. Zeolites have also been explored as suitable hosts for the encapsulation of drug molecules, in search for efficient drug delivery sysytems. Both zeolites and drugs have been administrated simultaneously to a patient without loss of the individual pharmacological effect of the drugs [164, 167]. Coating or impregnating zeolite with metallic silver nanoparticles to prepare zeolite composites can enhance the antibacterial ability of materials, and these materials can inhibit bacterial growth effectively [168]. It has been reported that silver embedded zeolite A was found to be antibactrerial against *E. coli*, *Bacillus subtilis* and *staphy‐ lococcus aureus* [165]. Moreover, polymer composites of plasticized poly (vinylchloride) pellets with silver zeolites demonstrated activity against *S. epidermidis* and *E. coli*, while polyurethane composites with silver zeolites showed antimicrobial action against *E. coli* and polylactid acidpolylactide (PLA)/silver zeolite composites also presented activity against *S. aureus* and *E. coli*, with silver being effectively released from the films [169].

**8. Antibacterial activity of carrier systems for intracellular infection**

therapeutic efficiency in the treatment of intracellular infections [31].

Tuberculosis, caused by *Mycobacterium tuberculosis*, is a ordinary lung infection that is even endemic to specified regions. Its prevalence has increased recently because it is often associated with AIDS. The *Mycobacterium avium* complex (MAC) complex is the main cause of hardships in immunodepressed patients [186]. There are drugs that are efficient against tuberculosis, but these are used in extended treatment, increasing the risk of side effects [187]. Moreover, tuberculosis has emerged as an occupational disease in the health care set-up. Although an effective therapeutic regimen is available, patient non-compliance (because of the need of taking antitubercular drugs daily or several times a week) results in treatment failure as well as the emergence of drug resistance [188]. The use of delivery systems facilitates the selective shuttling of antibiotic to the site of infection and such systems provide slow and prolonged drug release, which permits administration over longer intervals of time [189]. The encapsu‐ lation of antitubercular drugs in polymeric particles is another strategy to improve the current therapeutic regimen of tuberculosis. In the last few years several antitubercular drugscontaining PLGA and PLA microparticles and mainly nanoparticles have been comprehen‐ sively studied [190]. Fawaz et al*.* encapsulated the synthetic drug ciprofloxacin in polyisobutylcyanoacrylate (PIBCA) nanoparticles. When testing these nanoparticles against a *M. avium* infection in a human macrophage culture, it was found that though nanoparticle associated ciprofloxacin was more effective than unbound ciprofloxacin, it was much less so

**8.1. Infections due to mycobacteria**

Treatment of intracellular bacterial infection remains both a medical and economic challenge. Pathogens thriving or maintaining themselves in cells, or simply taking transient refuge therein, are indeed shielded from many of the humoral and cellular means of defense. They also seem more or less protected against many antibiotics [182]. Various infectious diseases are caused by facultative organisms that are able to survive in phagocytic cells. The intracel‐ lular location of these microorganisms protects them from the host defence systems and from some antibiotics with poor penetration into phagocytic cells. Intracellular infections are especially difficult to eradicate because bacteria fight for their survival using several ingenious mechanisms: inhibition of the phagosome–lysosome fusion, resistance to attack by lysosomal enzymes, oxygenated compounds and defensins of the host macrophages, escape from the phagosome into the cytoplasm [183]. Thus, the need for the development of improved antimicrobial chemotherapeutics and prophylaxis strategies is increasing [4]. In spite of the availability of a wide variety of *in vitro* active antibiotics, therapeutic deficiencies are reported, mainly because of the inability of the drugs to reach the bacteria harboring intracellular compartments or to perform their activity in the intracellular environments [182, 183]. However, the poor cellular penetration limits these use in the treatment of infections caused by intracellular pathogens [183]. One strategy utilized to improve the penetration of antibiotics into phagocytic cells is the use of carrier systems that deliver these drugs directly to the target cells [185]. Several *in vivo* and *in vitro* studies have reported the potential applications of various carrier systems to enhance the selectivity of antibiotics for phagocytic cells and sustain

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#### **7.11. Quantum dots**

Quantum dots (QDs) are nanocrystals formed by semiconductor materials, showing attractive photophysical properties, containing high quantum yield, resistance to photobleaching, and harmonic photoluminescence, making them potentially powerful tools in a range of biomed‐ ical applications [170, 171]. QDs are typically in the size range between 1 nm and 10 nm, composed of groups II–VI (e.g., CdSe) or II–V (e.g., InP) elements of the periodic table. QDs are highly bright, photostable and possess high quantum yield [172]. Due to their very small size, they possess unique properties and behave in different way than crystals in macro scale [173]. Water-soluble QDs may be cross-linked to biomolecules such antibodies, oligonucleo‐ tides, or small molecule ligands to render them specific to biological targets [174]. A variety of techniques have been explored to label cells internally with QDs, using passive uptake, receptor-mediated internalization, chemical transfection, and mechanical delivery. QDs have been loaded passively into cells by exploiting the innate capacity of many cell types to uptake their extracellular space through endocytosis [175, 176]. Krauss group utilized CdSe/ZnS streptavidin-coated QDs to detect solitary pathogenic *E. coli* O157:H7 in phosphate buffer saline solution [177]. Biotinylated anti-*E. coli* O157:H7 distinguished streptavidin-coated QDs via famous avidin–biotin binding. Once treated, QD labeled antibody selectively targeted pathogenic *E. coli* O157:H7 over common lab strain *E. coli* DH5α. This assay represented 2 orders of magnitude more sensitivity than using an organic dye with minimal non-specific binding between the QDs and the bacterial cells [178]. Recently, Luo et al. reported that CDTe QDs coupled to a rocephin antibiotic complex exhibited antibacterial activity against *Escheri‐ chia coli* [179]. The mechanism for the antimicrobial activity of QDs is unclear, but it is possible that QDs can produce singlet O2, a source of free radicals, under irradiation. Heavy metal ion oxides can also form the QDs core and result in antimicrobial activity [180]. A recent and excellent review emphasized the application of bioconjugated quantum dots for the detection of food contaminants such as pathogenic bacterial toxins like botulinum toxin, enterotoxins produced by *Staphylococcus aureus* and *Escherichia coli* [181].
