**3.7 Optical sensors**

Glucose is considered as a major component of animal and plant carbohydrates in biological systems. It acts not only as a source of energy of the living cells but also as metabolic intermediate in the synthesis of other complex molecules. Furthermore, blood glucose levels are also an indicator of human health conditions: the abnormal amount of glucose provides significant information of many diseases such as diabetes or hypoglycemia. Accurate determination of glucose is very important in clinical diagnosing as well as in food analysis. To date, various sensors for glucose analysis have been reported, and among them, fluorophotometry was used widely owing to its operational simplicity and high sensitivity (Shang et al., 2008, Li et al., 2009, Shiang et al., 2009, as cited in Jin et al., 2011). Recent

Corn-based foods followed by fumonisin B2 (FB2).The problems and risks associated with fumonisin contamination have resulted in the development of precise, reliable and sensitive methods for its determination in corn and corn-based foods (Magan & Olsen, 2004, as cited in Silva et al., 2009). Therefore, the quality parameters in the analysis of FB1 and FB2 in cornbased products obtained with LC with fluorescence detector have been investigated (Silva et al., 2009). Furthermore, a comparison study between fluorescence detector (FD), mass spectrometry, and tandem mass spectrometry with a triple quadrupole (QqQ) analyzer using an electrospray ionization interface for the determination of fumonisin B1 and B2 in corn-based products has been performed. A comparative study of the three LC detectors, FD, single quadrupole, QqQ for the analysis of fumonisins in corn samples has been performed. The response achieved by the three detectors was sensitive enough to study the maximum contents established by the EU legislation. These LC detectors would be appropriate for quantification purposes but the acquisition of at least two transitions

Low-molecular-weight compounds have been used widely as animal drugs, food additives, and pesticides, to achieve maximum productivity and profits directly or indirectly through food products. However, residues of such low-molecular-weight compounds in food products have been proven to be detrimental to human health. Therefore, development of a microsphere-based competitive fluorescence immunoassay for the determination of hazardous low-molecular-weight compounds in food has been described (Zou et al. 2008). In this method, antigens are covalently bound to carboxy-modified microspheres to compete monoclonal antibody with low-molecular- weight compounds in food samples; mouse IgG/fluorescein isothiocyanate conjugate is used as the fluorescent molecular probe. Thus, the hazardous low-molecular-weight compounds are quantified using a multiparameter flow cytometer. This method has been evaluated using clenbuterol as a model compound. It has a sensitivity of 0.01 ng/mL with dynamic range of 0.01–100 ng/ mL, and the concentration of clenbuterol providing 50% inhibition (IC50) is 1.1 ng/mL. The main advantages of this method are its high efficiency, biocompatibility, and selectivity, as well as

The aspects of fluorescence spectroscopy that may have value for solving problems in food science and technology have been summarized in a review article by Strasburg & Ludescher (Strasburg et al., 1995). In this review article, the techniques described, which depend on the measurement of the intensity, energy and polarization of fluorescence emission, have been

Glucose is considered as a major component of animal and plant carbohydrates in biological systems. It acts not only as a source of energy of the living cells but also as metabolic intermediate in the synthesis of other complex molecules. Furthermore, blood glucose levels are also an indicator of human health conditions: the abnormal amount of glucose provides significant information of many diseases such as diabetes or hypoglycemia. Accurate determination of glucose is very important in clinical diagnosing as well as in food analysis. To date, various sensors for glucose analysis have been reported, and among them, fluorophotometry was used widely owing to its operational simplicity and high sensitivity (Shang et al., 2008, Li et al., 2009, Shiang et al., 2009, as cited in Jin et al., 2011). Recent

illustrated by examples taken from the food science and related literature.

achieved with QqQ provided a univocal identification.

ultralow trace sample consumption and low cost.

**3.7 Optical sensors** 

advances in the noble metal clusters open a promising field toward the development of a satisfying fluorescence probe. In this regard, recently biomolecule-stabilized Au nanoclusters were demonstrated as a novel fluorescence probe for sensitive and selective detection of glucose (Jin et al., 2011). The fluorescence of Au nanoclusters was found to be quenched effectively by the enzymatically generated hydrogen peroxide (H2O2). By virtue of the specific response, the present assay allowed for the selective determination of glucose in the range of 1.0×10-5 M to 0.5×10-3 M with a detection limit of 5.0×10-6 M. In addition, it has been demonstrated the application of the present approach in real serum samples, which suggested its great potential for diagnostic purposes. In comparison with previous approaches for glucose detection, this method required no complicated preparation procedure, and used only commercially available materials. It also exhibited environmentally friendly feature and good sensitivity. Furthermore, the present nanosensor possessed red emission and excellent biocompatibility, which presage more opportunities for studying the biological systems in future applications.

Recently, metalloprotein design and semiconductor nanoparticles have been combined to generate a reagent for selective fluorescence imaging of Pb2+ ions in the presence red blood cells (Shete et al., 2009). A biosensor system based on semiconductor nanoparticles provides the photonic properties for small molecule measurement in and around red blood cells. Metalloprotein design was used to generate a Pb2+ ion selective receptor from a protein that is structurally homologous to a protein used previously in this biosensing system. This designed protein demonstrates a highly sensitive and selective biosensor that can reversibly detect Pb2+ ions in aqueous solutions. The modularity of semiconductor nanoparticle-based biosensors has allowed metalloprotein design and different semiconductor materials to be combined to significantly improve the detection of soluble, exchangeable Pb2+ ion concentrations to address inefficient Pb2+ ion chelation therapy.

The development of artificial receptors for molecular recognition studies of zwitterion amino acids under the physiological conditions is a very important research area since it can help to understand the important roles of free amino acids in biological systems. In this regard, a new fluorescence macrocyclic receptor based on the Zn(II) complex of a C2 terpyridine and a crown ether has been developed for molecular recognition of zwitterion amino acids in water/DMF solution with remarkable selectivity towards L-aspartate (K = 4.5x104 M-1) and L-cysteine (K = 2.5x104 M-1) (Kwong et al., 2009).

Copper is an essential trace element, its deficiency is one of the causes of anemia, but it is toxic at higher concentration levels. The uptake of copper by human beings above a certain level is known to cause gastrointestinal catarrh, Wilson's disease, hypoglycemia, and dyslexia. Increases in copper concentration in water and plants have resulted from industrial and domestic waste discharge, refineries, disposal of mining washings, and the use of copper as a base compound for antifouling paints. Therefore, the trace copper content in water and food must be monitored on a daily basis. In this regard, a highly sensitive and selective optical sensor for the determination of trace amounts of Cu2+ based on fluorescence quenching has been developed (Aksuner et al., 2009). The sensing membrane was prepared by immobilization of a novel fluorescent Schiff base ligand 4-(1-phenyl-1-methylcyclobutane 3-yl)-2-(2-hydroxy-5-romobenzylidene) aminothiazole, on polyvinlyl chloride. The accuracy of the proposed sensor was confirmed by analyzing standard reference materials of natural water and peach leaves. The sensor was successfully applied for the determination of copper

Current Achievement and Future Potential of Fluorescence Spectroscopy 241

photophysical properties together with excellent biocompatibility make CPNPs ideal for bioimaging applications ranging from single-molecule tracking to in vivo tumor detection while pH-dependent dissolvability of calcium phosphate offers the possibility of timed co-

Fluorescent nanoparticles have attracted increasing research attention due to their promising applications covering electro-optics to bio-nanotechnology. In this regard, monodispersed water-soluble fluorescent carbon nanoparticles (CNPs) were synthesized directly from glucose by a one-step alkali or acid assisted ultrasonic treatment (Li et al., 2011). The results showed that the particle surfaces were rich in hydroxyl groups, giving them high hydrophilicity. The CNPs could emit bright and colorful photoluminescence covering the entire visible-to-near infrared (NIR) spectral range. In this study they conclude that combining free dispersion in water (without any surface modifications) and attractive photoluminescent properties, CNPs should serve as a promising candidate for a new type fluorescence marker, bio-sensors, biomedical imaging, and drug delivery for applications in

Recent advances in ultrasensitive protein biosensors have brought significant impacts to proteomics, biomedical diagnostics, and drug discovery (Zhu et al. 2001, as cited in Huang & Chen, 2008). Advanced nanoscale biosensors based on nanoparticles, nanowires, and other nanomaterials have been developed to detect various proteins with improved sensitivity, specificity, and reliability (Fu et al., 2007, as cited in Huang, 2008). Ultrasensitive fluorescence nanosensors can detect the fluorescence signal from a fluorescence tag bound specifically with a single target molecule, but the plain fluorescence intensity measurement can hardly discriminate against a nonspecifically bound tag. Therefore, an electrically modulated fluorescence protein assay that can detect specific fluorescence from a single molecule assembled on an Au nanowire by manipulating the molecule with an electrical potential applied on the nanowire have been developed (Suxian et al., 2008). In their study, they conclude that the simple electrically modulated fluorescence detection method can be generally applied to various bioassays. The essential requirement of the method is to selectively modulate the specific fluorescence from the target molecules by an external reference field, which can be achieved by electrical, optical, magnetic, mechanical, or

Inorganic nanomaterials have been widely used in biological and environmental fields such as bio-labeling, imaging, drug delivery, separation processes and optical sensing. In these applications, the size and the shape of nanomaterials have very important effects on their properties. Recently, much attention has been given to one- dimensional nanomaterials for building various sensors, due to its high activity, high surface-to-volume ratio, easy assembly in an array for the device and especial suitability for intracellular detection by inserting it into cell (Zhang et al., 2004; Park et al., 2007, as cited in Xu et al., 2011). Therefore, a fluorescence sensor for selective detection of Cu(II) realized by covalently immobilizing derivatives of rhodamine6G (R6G) on the surface of silicon nanowires (SiNWs) has been designed and fabricated (Xu et al., 2011) The fabricated SiNWs-based chemosensor can be electively used for detection of Cu(II) with Cu(II)-special fluorescence enhancement over other metal ions. The Cu(II) sensor exhibits a good selectivity and

delivery of drugs to control cell function.

bioscience and nanobiotechnology.

biochemical interactions etc.

sensitivity.

in tap water and tea samples. This study showed the application of a PCT dye for preparation of a new Cu2+ sensitive optical chemical sensor for the first time. The sensor shows a high selectivity and quick response for Cu2+ over other common metal ions.

Heavy metal pollution is a global problem and it causes threat to the environment and human beings. Among the different heavy metal ions, mercury has received considerable attention due to its highly toxic and bioaccumulative properties. It is released from coal burning power plants, oceanic and volcanic emissions, gold mining, and solid waste incineration. Mercury vapor lamps, fluorescent lamps, electrical switches, batteries, thermometers and electrodes are the second largest sources of mercury discharge to the environment. In this regard, an ultrasensitive and selective spectrofluorimetric determination of Hg(II) using 2,5-dimercaptothiadiazole (DMT) as a fluorophore was developed (Vasimalai & John, 2011). In this study, the practical application of the present method was demonstrated by determining Hg(II) in tap water, river water and industrial waste water samples. The obtained results have a good agreement with inductively coupled plasma atomic emission spectrometric (ICP-AES) and atomic absorption spectrometric (AAS) methods. According to the literature, this is the first report for the lowest detection with the highest selectivity for Hg(II) in a water medium by the fluorimetric method.
