**2. Strategies for selective target capture**

antibiotics and pesticides [1, 2]. It is urgent to develop rapid and sensitive strategy to classify, quantify and assess them from the environment, which is the basic information to early warning for their threats and getting the precondition for solving these problems. Environmental monitoring requires the analysis of pollutants at (very) low concentrations since many of the pollutants have serious consequence at even extremely low levels. In addition, the methods to

Different techniques including chromatography, spectroscopy, mass‐spectra methods are well established in environmental analysis [3–5], but most of these methods require sophisticated instruments and some of them are lack of sufficiently recognition capacity, which limit their wide applications for infield application. Recently, advanced nanomaterial‐based methods have contributed a lot to this area, such as microfluidics, electrochemical sensor, surface plasmon resonance (SPR) method, single‐molecule spectroscopy and hyperspectroscopy [6– 8]. Among these methods, surface‐enhanced Raman spectroscopy (SERS) is one of the most promising methods for environmental monitoring. As a molecular vibrational spectroscopy, SERS holds several outstanding advantages compared to the traditional techniques. Firstly, by making use of the SPR‐induced strong electromagnetic field to enhance signals of the analytes, SERS has high sensitivity which enables detection at low concentrations, even as low as to single molecule level [9]. Secondly, the obtained Raman spectra contains abundant molecular information of the analytes, and the finger‐print information is valuable for identification and classification [10]. Thirdly, the Raman character peaks have very narrow width, which enables multiple detection or complex identification. Unlike other vibrational spectroscopy such as infrared spectroscopy, SERS can be applied directly in solution since water has little back‐ ground signal. SERS technique is also compatible with different sample conditions including aqueous, solidary and even gaseous state, and it needs less sample preparation and preoper‐ ation. This technique can get the character signal within seconds to minutes, which is suitable for rapid signal readout. SERS is also very convenient and cost‐effective to be combined with miniaturized Raman spectrometers and offers good practical utility for real application, even

The above‐mentioned remarkable advantages have led many significant achievements of SERS in the environmental detection [11]. Recently, several review articles have covered different considerations, such as facing various targets such as organics, [12] ions [13] or pathogens [14]. Considering the real condition when performing SERS in environmental application, we believe that special attention should be paid to the following aspect in order to fully realize the potential of SERS method. (1) SERS phenomenon only takes effect when the analyte is near the surface of the SERS substrate, which usually needs to be within several nanometers [15]. While for most of the environmental targets, their interaction with the bare substrate is not strong enough to get them close to the substrate; thus, it is important to shorten the distance between them. (2) The matrix of the environmental samples is complex, which will interrupt the effective interaction between the substrate and the analytes, and hence, the proposed method should have specific selectivity to the interested targets. (3) In many cases, the proposed method should have the ability to concentrate the target in order to meet the demands of sensitivity. (4) For most of infield detections, a strategy is required to be compatible

be used should also be simple and rapid for their operations in the real application.

for infield detection.

332 Raman Spectroscopy and Applications

The SERS effect is known to be a localized first-layer effect. The observed enhanced Raman signal comes from the analytes close to the metallic structures with a distance no longer than 5 nm, and the signal intensity exponentially decreases with increasing the distance [15]. The analytes may be divided into two groups. In the first group, the analytes have strong affinity to the bare surface of metallic structures due to their functional groups such as amino (−NH2) and thiol (−SH) groups. These analytes are easily absorbed on the substrate, and their SERS responses are obtained directly if they are SERS active. As the second group of analytes, most of which are pollutants to be concerned, they have no strong affinity to the unmodified substrate, and hence, the distance between the analyte and the SERS substrate is too large to producing enough SERS-enhancing effect. Therefore, it is important to functionalize the surface of the substrate to capture such weakly affinitive targets. In addition, it should be noted that many SERS substrates are made with wet chemical synthesis methods. These wet chemically synthesized SERS substrates usually contain one or more surfactants and/or other organic species being used as the shape control reagents and/or reducing agents during the synthesis. The presence of these species may hinder the effective contact of analytes with the substrate surface, and hence, surface replacement of these capping species is needed to avoid the interference and further enhance the capture capacity of the substrate for the specific targets. Concerning targets with weak Raman response, surface functionalization can be used to increase the sensitivity by producing special interaction or generating specific complexes between analytes and modified molecules which could be used for the indirect detection [16, 17]. The detailed measures may aim at one or more of the following five types of interaction enhancement.

#### **2.1. Electrostatic or hydrophobic interaction**

Different function structures or molecules are used to enrich targets from the matrix, one of the functions is to modify the SERS substrate surface with improved electrostatic or hydrophobic interactions. For example, in most common colloidal systems (such as citrate reduced Ag or Au NPs), the substrate surface is negatively charged. To enhance the attractive interaction between the particle surface and the negatively charged target molecules, a substrate with controllable surface charge is favorable. By employing aliphatic amino acids as reductant and modifier, controllable surface charge range from −60 to +30 mV was obtained, which is favorable to smaller electrostatic repulsion and even attraction to increase analyte retention [18]. Besides, the SERS substrate are often hydrophilic, but various toxic organic pollutants bearing aromatic structures are highly hydrophobic. Polycyclic aromatic hydrocarbons (PAHs) are a family of these pollutants, which consist of fused aromatic rings and contain no sub‐ stituent that can absorb to the hydrophilic surface. The affinity between such hydrophobic analytes and the hydrophilic substrate may be enhanced by making use of the hydrophobic interaction. For example, Jing et al. reported thiol‐functionalized magnetic nanoparticles (NPs) for the SERS detection of eight kinds of PAHs including benzene and naphthalene with limits of detection (LODs) down to 10−7 mol L−1 [19]. Similarly, alkyl dithiol was modified onto the metal particles to enhance the affinity of pesticides to the substrate, and this greatly promoted adsorption constant and led to the LOD down 10−8 mol L−1, proving a solid basis for identifi‐ cation and quantitative analysis of organochlorine pesticides [20]. For the detection of aromatic organics, π‐π stacking could also be used, where the modifying molecules could be aromatic molecules and also special materials like graphene or carbon nitride with absorption ability [21, 22]. The capture strategies by making use of electrostatic or hydrophobic interaction could effectively enhance the detection activity to targets, but the selectivity is still weak due to the low selectivity of these interactions.

#### **2.2. Forming surface complex**

Surface modification with molecules that can selectively bond to the target by forming a complex is an effective method to enable selective detection. For example, mercury ion (Hg(II)) is one of the most toxic pollutants with bioaccumulative activity. It holds weak SERS response and weak interaction to the common SERS substrate, and thus, it is difficult to be detected by SERS directly. By modifying the gold nanomaterials with tryptophan (a SERS‐active molecule that can interact with Hg(II) to form a complex with a weak SERS response), an easy and highly selective method was proposed to recognize Hg(II) with the LOD down to 5 ppb level [23]. By making use of the specific interaction between Hg(II) and single‐stranded DNA to convert into a hairpin structure through forming of thymine–Hg(II)–thymine complex, Hg(II) ions at concentrations as low as to 0.2 ppt (1p mol L−1) were readily discriminated, being much lower than conventional analytical methods (usually nanomolar level) [24]. Due to the high binding specificity of DNAzyme to Pb2+ ions, a SERS DNAsyme biosensor was developed to detection of Pb2+, such detection was further accomplished by SERS nanoprobe labeled with both DNA and Raman probe for signal amplification [25]. Another example of making use of the surface complex is the SERS detection of trinitrotoluene (TNT) with cysteine. Cysteine‐ modified gold nanoparticles (NPs) could selectively recognize of TNT molecules due to the formation of Meisenheimer complex, which underwent aggregation via electrostatic interac‐ tion to form hot spots and further enhanced the Raman signal of the complex by nine orders. High sensitivity (low to 2 pico molar level) and selectivity were observed for the detection of TNT without dye tagging [26]. Similarly, the surface‐modified gold NPs with (aminometh‐ yl)phosphoric acid were synthesized to selectively capture uranium(U) ions by making use of its phosphonic tails as terminal group. Without any pretreatment, the proposed method was performed directly for detection of uranium in contaminated water even under low pH and high salts conditions [27].

#### **2.3. Host–guest interaction**

[18]. Besides, the SERS substrate are often hydrophilic, but various toxic organic pollutants bearing aromatic structures are highly hydrophobic. Polycyclic aromatic hydrocarbons (PAHs) are a family of these pollutants, which consist of fused aromatic rings and contain no sub‐ stituent that can absorb to the hydrophilic surface. The affinity between such hydrophobic analytes and the hydrophilic substrate may be enhanced by making use of the hydrophobic interaction. For example, Jing et al. reported thiol‐functionalized magnetic nanoparticles (NPs) for the SERS detection of eight kinds of PAHs including benzene and naphthalene with limits of detection (LODs) down to 10−7 mol L−1 [19]. Similarly, alkyl dithiol was modified onto the metal particles to enhance the affinity of pesticides to the substrate, and this greatly promoted adsorption constant and led to the LOD down 10−8 mol L−1, proving a solid basis for identifi‐ cation and quantitative analysis of organochlorine pesticides [20]. For the detection of aromatic organics, π‐π stacking could also be used, where the modifying molecules could be aromatic molecules and also special materials like graphene or carbon nitride with absorption ability [21, 22]. The capture strategies by making use of electrostatic or hydrophobic interaction could effectively enhance the detection activity to targets, but the selectivity is still weak due to the

Surface modification with molecules that can selectively bond to the target by forming a complex is an effective method to enable selective detection. For example, mercury ion (Hg(II)) is one of the most toxic pollutants with bioaccumulative activity. It holds weak SERS response and weak interaction to the common SERS substrate, and thus, it is difficult to be detected by SERS directly. By modifying the gold nanomaterials with tryptophan (a SERS‐active molecule that can interact with Hg(II) to form a complex with a weak SERS response), an easy and highly selective method was proposed to recognize Hg(II) with the LOD down to 5 ppb level [23]. By making use of the specific interaction between Hg(II) and single‐stranded DNA to convert into a hairpin structure through forming of thymine–Hg(II)–thymine complex, Hg(II) ions at concentrations as low as to 0.2 ppt (1p mol L−1) were readily discriminated, being much lower than conventional analytical methods (usually nanomolar level) [24]. Due to the high binding specificity of DNAzyme to Pb2+ ions, a SERS DNAsyme biosensor was developed to detection of Pb2+, such detection was further accomplished by SERS nanoprobe labeled with both DNA and Raman probe for signal amplification [25]. Another example of making use of the surface complex is the SERS detection of trinitrotoluene (TNT) with cysteine. Cysteine‐ modified gold nanoparticles (NPs) could selectively recognize of TNT molecules due to the formation of Meisenheimer complex, which underwent aggregation via electrostatic interac‐ tion to form hot spots and further enhanced the Raman signal of the complex by nine orders. High sensitivity (low to 2 pico molar level) and selectivity were observed for the detection of TNT without dye tagging [26]. Similarly, the surface‐modified gold NPs with (aminometh‐ yl)phosphoric acid were synthesized to selectively capture uranium(U) ions by making use of its phosphonic tails as terminal group. Without any pretreatment, the proposed method was performed directly for detection of uranium in contaminated water even under low pH and

low selectivity of these interactions.

**2.2. Forming surface complex**

334 Raman Spectroscopy and Applications

high salts conditions [27].

The above‐mentioned strategies for detection of targets by forming surface complex can greatly increase the sensitivity and selectivity, and the key point is to find a specific interaction between the analyte and the modifier; such analyte–receptor systems are not limited to the complex formation, but also many other interactions, such as host–guest interaction, molecular imprinting (MIP) recognition and antibody–antigen interaction. These interactions all have been successfully applied into promoting the selectivity of SERS‐based method into specific target monitoring [28–31].

**Figure 1.** Molecular structure of β‐CD.

Host–guest interaction is an important phenomenon in supramolecular chemistry, which describes the special interaction between the host molecules with unique structure and the guest smaller molecules or ions. It encompasses the idea of molecular recognition and interaction through noncovalent bonding (such as hydrogen bonds, ionic binds, van der Waals forces and hydrophobic interactions) [32]. It has been widely used in the drug delivery, the removal of hazardous materials from the environment and for sensing called indicator–spacer– receptor approach [33]. It also can be used for the SERS detection of the specific guest molecules by making use of the host structures as a modifier. Several typical host molecules have been applied as functional modifiers, such as viologen host lucigenin for the selective detection of PAHs or pesticides [34–36], dithiocarbamate calix [4] arene derivatives for the capture and detection of organic pollutants such as pyrene and PAHs [28, 37], and cucurbit[n]uril for the SERS monitoring of diaminostilbene [38]. Cyclodextrins (CDs) are another class of host molecules in supramolecular chemistry, and they have cyclic oligosaccharide structures with hydrophobic internal cavities which can selectively capture suitable nanosized guest mole‐ cules [39]. The most widely used cyclodextrin is β‐cyclodextrin (β‐CD) with seven glucose units, which is a natural product of specific bacteria. As shown in **Figure 1**, the cavity diameter is 6.4 À, which contributes to the interaction with guest molecules [39].

**Figure 2.** Molecular interaction between SMM and CD. Reproduced with permission from Ref. [45].

As a functional modifier, β‐CD has been used for the SERS detection of both organic pollutants and inorganic ions, such as polychlorinated biphenyls (PCB‐77, PCB‐1) down to 3 μM, [40] methyl parathion at picomolar level, [41] PAHs (anthracene, pyrene or anthrecene) [39, 42] and micromolar Pb2+ ions [29]. In most of the reported works, thio‐modified β‐CD (such as per‐6‐ deoxy‐(6‐thio)‐β‐CD) was used because of the weak modifying efficiency of natural β‐CD onto the surface of the metal structure. In our earlier work, we proposed an in situ reduction strategy to synthesize β‐CD modified Ag nanoparticles by making use of the reducing activity of the natural β‐CD under heat and alkaline condition [43, 44]. The obtained substrate was success‐ fully applied to detection of sulfonamide antibiotics with the LOD as low as 10 ng mL−1 [45]. Our results showed that by employing β‐CD as both reductant and shape‐controlling agent, the β‐CD‐modified Ag NPs could be easily obtained with controllable size and distribution, and much enhanced detection ability was observed with enhancement factor (EF factor) up to 1.97 × 106 . The mechanism for the promoted recognition ability was further studied by fluorescence and 1 H NMR methods. As shown in **Figure 2a**, the character fluorescence emission at 445 nm of sulfamonomethoxine (SMM) solution was significantly decreased with the addition of β‐CD, and such a quenching effect suggests a considerably strong interaction between SMM and β‐CD. More detailed information for the interaction from the molecular level was obtained from NMR analysis. **Figure 2b** shows the 1 H NMR spectra of SMM, β‐CD and SMM‐β‐CD complex. After assigning the character chemical shift to each proton, obvious shift of several protons was observed, such as H‐a, H‐e and H‐f of SMM by 0.01–0.03 ppm, and H‐1, H‐4, OH‐2, OH‐3 and OH‐6 protons by 0.01–0.10 ppm. In order to get insight into the exact interaction sites, we further performed the 2D NMR characterization which could not only give the chemical shifts similar to 1 H NMR, but also reveals the correlation between the interacted protons. The obtained 2D NOESY NMR spectra of the SMM‐β‐CD complex are shown in **Figure 2c**. As indicated in the spectra, these cross‐peaks indicate a close interaction of β‐CD with SMM, since such cross‐peaks could only be observed when the distance of relative molecules is shorter than 0.5 nm, such as H‐3 with H‐d, H‐1 with H‐f, these cross‐peaks comes from the protons of SMM and interior cavity of β‐CD, as marked by elliptic circles. Besides that, cross‐peak from the SMM with the outside face of β‐CD cavity was also obtained, as marked by rectangles in **Figure 2c**. These results provide us the interaction pattern of β‐CD with SMM: It could not only set SMM into its cavity by host–guest interaction, but it also acts as a scaffold or a bridge to pull the SMM close to the out‐surface of the cavity which yield greatly enhanced activity in SERS detection of SMM. The strategy of using native β‐CD as scaffold for analytes with low affinity to the substrate shows wide application prospect in selectively capture and sense other concerning pollutants.

#### **2.4. Antibody–antigen interaction**

**Figure 2.** Molecular interaction between SMM and CD. Reproduced with permission from Ref. [45].

1.97 × 106

fluorescence and 1

336 Raman Spectroscopy and Applications

As a functional modifier, β‐CD has been used for the SERS detection of both organic pollutants and inorganic ions, such as polychlorinated biphenyls (PCB‐77, PCB‐1) down to 3 μM, [40] methyl parathion at picomolar level, [41] PAHs (anthracene, pyrene or anthrecene) [39, 42] and micromolar Pb2+ ions [29]. In most of the reported works, thio‐modified β‐CD (such as per‐6‐ deoxy‐(6‐thio)‐β‐CD) was used because of the weak modifying efficiency of natural β‐CD onto the surface of the metal structure. In our earlier work, we proposed an in situ reduction strategy to synthesize β‐CD modified Ag nanoparticles by making use of the reducing activity of the natural β‐CD under heat and alkaline condition [43, 44]. The obtained substrate was success‐ fully applied to detection of sulfonamide antibiotics with the LOD as low as 10 ng mL−1 [45]. Our results showed that by employing β‐CD as both reductant and shape‐controlling agent, the β‐CD‐modified Ag NPs could be easily obtained with controllable size and distribution, and much enhanced detection ability was observed with enhancement factor (EF factor) up to

. The mechanism for the promoted recognition ability was further studied by

at 445 nm of sulfamonomethoxine (SMM) solution was significantly decreased with the addition of β‐CD, and such a quenching effect suggests a considerably strong interaction between SMM and β‐CD. More detailed information for the interaction from the molecular

and SMM‐β‐CD complex. After assigning the character chemical shift to each proton, obvious shift of several protons was observed, such as H‐a, H‐e and H‐f of SMM by 0.01–0.03 ppm, and H‐1, H‐4, OH‐2, OH‐3 and OH‐6 protons by 0.01–0.10 ppm. In order to get insight into the exact interaction sites, we further performed the 2D NMR characterization which could not

level was obtained from NMR analysis. **Figure 2b** shows the 1

H NMR methods. As shown in **Figure 2a**, the character fluorescence emission

H NMR spectra of SMM, β‐CD

Antibody–antigen interaction is the most specific and useful recognition interaction that has been widely used in clinical diagnose. The highly specific binding is due to the specific chemical constitution of each antibody. The antigenic determinant or epitope is recognized by the paratope of antibody, situated at variable regions of polypeptide chain which also has unique hypervariable regions in each antibody [46]. The strategy of using antibody as modifier for selective SERS detection of antigens is commonly applied in the biological detection and imaging, and also for environmental monitoring, and it is widely used for the detection of pathogen organisms with both label and label‐free methods [47, 48]. There are several reviews that summarized the achievement of such strategy in the pathogen detection [49–51]. Beside the common antibody modification, other interactions that base on immunological recognition has also been applied into the detection of specific pollutants recently. For example, aptamer is made of single‐stranded DNA oligomer that can be selected against specific target (biological macromolecule or small organic molecule) come from systematic evolution of ligands by exponential enrichment (SELEX) [31]. On the basis of capture and enrichment by the PCB‐77 specific aptamer, the improved detection of PCB77 was accomplished with a LOD down to 1 × 10−8 mol L−1 [52]. The aptamer‐based strategy was used to detect bisphenol A, one of the most important endocrine disrupting chemicals, with a LOD as low as 3.9 pg/mL with excellent recovery for real sample detection [53]. Aptamer‐based SERS sensor has also been developed for the selective detection of Hg2+ by employing the structure‐switching aptamer in the presence of spermine [54]. These antibody, aptamer or other recognition structures such as phage that making use of the immunological reaction have the advantage of high selectivity, but also have some limitations that need further study, due to their low stability under harsh conditions.

#### **2.5. Artificial antibody–antigen interaction (molecular imprinting effect)**

Molecular imprinting (MIP) is a powerful technique to create specific recognition site in polymer by employing target molecules or molecules with similar structures as templates. The recognition cavities after removing the templates provide the capability and functionality to selectively rebind specific targets without interference from other molecules. The surface MIP technique has been used for selective detection and removing of organic pollutants from the complex matrix [30, 55]. Such a strategy could be performed to functionalize the surface of the substrate to promote the recognition ability of the SERS method. Zhang et al. proposed a ligand replacement approach to rapid determination of penicilloic acid in the penicillin by using a molecularly imprinted monolayer as recognition surface, which efficiently excluded the interference of penicillin and provided a selective determination down to 0.1‰ (w/w) [56]. Similarly, thiol‐terminated MIP microspheres which have been immobilized on a gold‐coated substrate were used for the selective capture of nicotine, showing good capture efficiency and SERS response [57]. In considering of using MIPs techniques for selective SERS detection, two key points should be kept in mind. The first is that the imprinted layer should be thin thick because a thick layer will interfere the enhancement from the under layer metal structure. The second is that the polymer film should have clear background signal to prevent any interfer‐ ence to the detection of target molecules.
