Keywords

4.1 Introduction

The recognition and sensing of chemical and biologically important species have emerged as significant goals in the field of nanosensors in recent years (Quang and Kim 2010; Zhou et al. 2011; Kim et al. 2011). Nanostructured materials are of great interest due to their size- and shape-dependent physical and chemical properties (Ferrando et al. 2008; Huynh et al. 2002; Kumar et al. 2016). There is also an increased interest in the synthesis of more complex nanostructures such as core-shell and hollow particles for advanced applications in chemical and biological sensing (Liang et al. 2009; Taton et al. 2000). The design and fabrication of nanomaterial-based sensors have generated great interest in detection of chemical and biological important target species with high precision and accuracy (Awual et al. 2015; Borisov and Wolfbeis 2008; Jung et al. 2010). The detection of the targeted species typically occurs through a controlled binding event and is then transmitted as a readable signal. A variety of signaling procedures are used for nanoparticle-based sensor with light absorption or either through fluorescence (Descalzo et al. 2005; Doleman et al. 2007) or scattering or current and potential changes (Zhan and Bard 2007).

Several analytical techniques, such as resonance Rayleigh scattering (Wen et al. 2013; Zhan et al. 2012), atomic absorption spectrometry (AAS) (Gao et al. 2012), inductively coupled plasma mass spectrometry (ICP-MS) (Chen et al. 2010), cold vapor atomic fluorescence spectrometry (Zhang et al. 2010), attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) (Vigano et al. 2005), etc., have been developed for sensing chemical and biological molecules. Most of the analytical techniques are more expensive and time-consuming processes. Consequently, high selectivity and sensitivity with simple instruments and easy operation have received much attention in recent years. Absorption spectroscopy (colorimetric/UV-visible), fluorescence spectroscopy, Raman spectroscopy, and electrochemistry are powerful analytical techniques for qualitative and quantitative sensing of chemical and biological molecules, as they offer simple handling, easy interpretation, moderate cost, portability, and fast analysis of the samples.

In general, sensors involve interaction between the target molecule (analyte) and a receptor (chemical or biological) that is signaled by an easily detectable change (Fabbrizzi and Poggi 1995). Most sensors depend on the binding mechanism or a chemical reaction to change the reporter characteristics (De Silva et al. 1997). All the sensors have been synthesized along these strategies for the detection of chemical and biological important analytes. According to the current IUPAC’s definition “A chemical or bio-sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal.” Optical reporter absorption (colorimetric and UV-visible), fluorescence , Raman spectroscopy, and electrochemical sensing have been widely used in this context (De Silva et al. 2009; Nguyen and Anslyn 2006), owing to a usually fast and convenient implementation. The instrumentation required for the use of such techniques is relatively simple and cheap which makes the quite attraction of sensors. However, the development of sensors is not trivial; materials science, molecular recognition, and device implementation are some of the aspects that play a significant role in the design of sensors.

In order to obtain the excellent selectivity and high sensitivity of chemical and biosensing, specific recognition and/or signal triggering elements are introduced which should be functionalized on the surface of nanomaterials with appropriate methods. The various approaches (Fig. 4.1) for the functionalization of nanoparticles (NPs) usually include noncovalent interaction such as physical adsorption, specific affinity interaction, and entrapment of chemical or biomolecules around the nanoparticles and covalent interaction of chemical or biomolecules with the functional groups on the nanoparticle surface (Veiseh et al. 2010). In signal amplification strategies, the nanomaterials usually act as catalysts to trigger the detectable signal or carriers for high loading of signal tags.

Fig. 4.1
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Schematic illustration of signal amplification strategies using unmodified and surface-functionalized nanomaterials for sensing via absorption, fluorescence, SERS and electrochemical methods

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4.2 Principle and Operation Stages of a Sensor (Fig. 4.2)

Fig. 4.2
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Sensors principle and operation stages of a sensing system

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4.3 Structure Adopted for Nanomaterial-Based Chemo- and Biosensors

The development of nanomaterial-based chemical and biological sensors is typically under direct spectroscopic and reagent-mediated sensors using both unmodified and surface-functionalized nanomaterials. This chapter exclusively deals with powerful analytical techniques such as absorption spectroscopy (colorimetric/UV-visible), fluorescence spectroscopy, Raman spectroscopy, and electrochemistry. The detection of analyte is directly based on some fundamental optical or electrical property via color changes/absorption, emission, scattering, or current and/or potential changes. The whole structure of this chapter is shown in Fig. 4.3.

Fig. 4.3
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Sensing platform for nanomaterial- based chemo- and biosensors

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4.4 Nanomaterials as Sensing Platforms

The development of nanomaterial-based sensors is usually achieved either in unmodified or in functionalized nanoparticles.

4.4.1 Unmodified Nanoparticles

Unmodified metallic nanoparticle surface has positive charge with flexible high affinity toward negatively charged ones. This affinity differences offer many advantages; since different sizes and shapes of metallic nanoparticle solution have different stability, they give various color signals.

4.4.2 Functionalized Nanoparticles

Nanoparticles can be converted into powerful nanoscale chemo- and biosensors by functionalizing their surface with specific binding receptors and/or reporter molecules such as nucleic acids, proteins, dyes, and fluorescent tags. In addition, many other specific organic compounds which introduce new functionality will enhance their selectivity and sensitivity of target molecules.

4.5 Sensing Methods

In general, chemo- and biosensors may be categorized under two types: (i) direct sensors and (ii) reagent-mediated sensors.

4.5.1 Direct Spectroscopy Sensing

In a direct sensor, the analyte is detected directly via the basic phenomenon; localized surface plasmon resonance (LSPR) is responsible for the brilliant colors exhibited by the metal nanoparticles under illumination. Various direct sensor techniques are widely used, but this chapter exclusively deals with UV-visible and fluorescence spectroscopic methods and Raman/surface-enhanced Raman spectroscopy (SERS) measurements .

4.5.2 Reagent-Mediated Sensing

In a reagent-mediated sensing system, the change in analytical response is coming from the intermediate reagent. For example, analyte concentration is monitored from the analyte sensitive dye molecule or catalyst. This chapter mainly deals about only two types of reagent-mediated sensors, namely, colorimetric sensing and electrochemical sensors .

4.6 Analytical Techniques and Signals

4.6.1 UV-Visible Absorption-Based Sensors

UV-visible absorption spectroscopy is an important analytical technique for sensor application, as it offers simple handling, easy interpretation, moderate cost, portability, and fast analysis of the samples. To design nanomaterial- based sensing platform for the detection of environmental pollution materials (heavy and biological essential metal ions) and biomolecules, researchers have utilized the nanoparticle aggregation- or disaggregation-induced color change that reflects a redshift in the extinction spectrum (Polavarapu et al. 2014; Zhai et al. 2014). Based on this principle, simple and rapid colorimetric detection of Cu2+ ions in aqueous medium via aggregation of thiomalic acid-functionalized Ag nanoparticles (TMA-AgNPs) has been developed as shown in Fig. 4.4A (Tharmaraj and Jyisy 2014). A visually detectable color change from yellow to purple in the presence of Cu2+ ion over other metal ions was observed (Fig. 4.4B). The aggregation behavior was confirmed by SEM images of TMA-AgNPs and TMA-AgNPs in the presence of 100 nM, 500 nM, and 1000 nM Cu2+ which is shown in Fig. 4.4C. TMA-AgNPs can be aggregated only in the presence of Cu2+ that cause a redshift of SPR band from 392 to 423 nm as observed in UV-visible spectra (Fig. 4.4D). Sensitivity response of TMA-AgNPs to Cu2+ was estimated in the concentration ranges of Cu2+ from 0.25 nM to 1000 nM, and their corresponding UV-visible spectra are shown in Fig. 4.4E. This chemosensor has excellent selectivity and sensitivity for Cu2+, and the detection limit is as low as 0.25 nM and also successfully applied for the detection of copper ion in real water samples.

Fig. 4.4
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(A) Schematic diagram for the interaction of TMA-AgNPs with Cu2+. (B) Photograph of TMA-AgNPs (0.25 mM) in presence of Cu2+ and various metal ions (500 nM). (C) SEM images of TMA-AgNPs (a), TMA-AgNPs in the presence of 100 nM (b), 500 nM (c), and 1000 nM (d) of Cu2+ ions. (D) UV-visible spectra of TMA-AgNPs (0.25 mM) in the presence of Cu2+ and various other metal ions (100 nM) in aqueous medium. (E) UV-visible spectra of TMA-AgNPs (0.25 mM) upon addition of Cu2+ from 0.25 to 1000 nM in aqueous medium

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A simple, selective, and sensitive colorimetric detection of Hg2+ was achieved in aqueous medium using soaproot plant-stabilized silver nanoparticles by biologically green synthesis (Farhadi et al. 2012). In the presence of Hg2+, the yellow AgNP solution was turned to colorless, accompanying the broadening and blueshifting of SPR band. This sensing method has been applied for real sample analysis, and the detection limit is as low as 2.2 μM. The selective detection of Co2+ ion on different shapes of (nanosphere, nanoplate, and nanorod) glutathione-modified silver nanoparticles (GSH-AgNPs) is based on aggregation by the formation of chelating complex between Co2+ ion and COO− groups of glutathione (Sung et al. 2013). Selective colorimetric detection for Cu2+- and lidocaine hydrochloride (LC-HCl)-based sensor was developed by the aggregation of homocysteine-functionalized silver nanoparticles (Dou et al. 2013) that result in the color change from deep brown to bright yellow, and the SPR intensity characterized at 571 nm was found to be proportional to the concentration of Cu2+ ions, and the detection limit is as low as 3.2 nM. “Mix-and-detect” rapid virtual colorimetric ultrasensitive detection of Hg2+ ion using label-free cysteamine-capped AgNPs has been developed (Bhattacharjee and Chakraborty 2014). In the presence of Hg2+ ion, thiophillic Hg2+ would lead to partial exchange of cysteamine ligand with the detection limit of 275 pM. Some recent reports for UV-visible absorption method-based sensing systems are listed in Table 4.1.

Table 4.1 List of UV-visible absorption-based colorimetric sensors
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4.6.2 Fluorescence-Based Sensors

Fluorescence spectroscopy is one of the most powerful ultrasensitive analytical techniques when compared to other analytical methods which make the detection of single molecules. Also this approach provides a limit of detection (LOD = 3.3σ/S, where σ is standard deviation of the response and S is the slope of the calibration curve) at a signal-to-noise (S/N) ratio for the detection up to parts-per-trillion level with good precision and accuracy (Wang et al. 2014; Nolan and Lippard 2008). Design of nanomaterial- based fluorescent sensors for quick detection of targets molecules has been developed either by fluorescent enhancement (turn on) or quenching (turn off) that controls the response through both energy and electron transfer processes (Liu et al. 2014; Jia et al. 2014).

Alginate-stabilized silver nanocubes (AgNCbs) bound to rhodamine 6G (Rh6G) composite as selective sensor for Hg2+ ion in aqueous solution have been developed (Tharmaraj and Pitchumani 2011) with the detection limit as low as 0.1 nM. Rh6G dye molecules bound to AgNCbs surface were observed in quenching the fluorescence; when there is presence of Hg2+ ion, bound Rh6G is released from the alginate-AgNCb surfaces which indicate a large fluorescence restoration with a concomitant color change from yellow to purple. Ratiometric fluorescence probe is designed and developed (Niu et al. 2015) by linking with two parts, positively charged aggregation-induced emission (AIE) organic fluorescence nanoparticles (OFNPs) as the reference and negatively charged Au nanoclusters (Au NCs) as the response. This probe can be used to detect Hg2+ and also melamine, since red fluorescence of Au NCs can be quenched by mercury ions and recovered by melamine, due to the strong affinity between metallophilic Hg2+–Au and Hg2+–N. This dual-emission ratiometric fluorescence probe has good biocompatibility; hence it is applicable for biological imaging and detection.

Thiol-DNA-functionalized gold nanoparticles (AuNPs) as a simple fluorescence spectrometric sensor for Hg2+ detection in aqueous solution were developed (Wang et al. 2015). Hg2+ is selectively induced conformational change of single-stranded DNA (ssDNA) with an enhanced fluorescence resonance energy transfer (FRET) process between the energy donor (fluorescein, FAM) and the energy acceptor (AuNPs). The assay enables the detection limit as low as 8 nM and also applied to monitor Hg2+ in tap water samples. Noncross-linking aggregation of fluorosurfactant-capped silver nanoparticles for colorimetric sensing of cysteine was developed (Chen et al. 2014). High specificity toward cysteine was observed as the nonionic fluorosurfactant ligand thiol-silver interaction prohibited the binding of other functional groups on the surface of AgNPs. This sensing probe can be successfully applied for the determination of cysteine in human urine and plasma samples with the detection limit as low as 0.05 μM.

Dansyl fluorophore-functionalized thiol-stabilized AgNPs containing 2-aminothiophenol units with excellent selective binding sites for Cu2+ ion were developed, and energy transfer (ET) from the dansyl moiety to the copper complex occurs that causes the fluorescent ratiometric response as seen in Fig. 4.5A (Tharmaraj and Pitchumani 2013). High-resolution transmission electron microscopy (HRTEM) images of highly dispersed thiol-stabilized silver nanoparticles, 2D lattice fringe spacing image, and selected area electron diffraction (SAED) pattern are shown in Fig. 4.5B(a-d). The particle size and lattice fringes of AgNP were 5.5 nm and 0.2 nm, respectively. The lattice planes of nano silver were observed in SAED pattern. Selectivity of fluorescent ratiometric response for the decrease in fluorescence at 497 nm and an increase in fluorescence at 410 nm with an isoemissive point at 445 nm was favored only in the presence of Cu2+ ions, whereas other metal ions led to no fluorescence ratiometric change as seen in Fig. 4.5C. Sensitivity response of dansyl-AgNPs to Cu2+ was evaluated in the concentration ranges of Cu2+ from 0.5 nM to 0.5 μM, and their corresponding fluorescence spectra is shown in Fig. 4.5D.

Fig. 4.5
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(A) Schematic diagram proposed binding mechanisms for dansyl-AgNPs with Cu2+ ion. (B) (a), (b) HRTEM images of highly dispersed thiol-stabilized AgNPs. (c) The 2D lattice fringes of HRTEM image. (d) Selected area diffraction pattern is showing the corresponding planes. (C) Fluorescence spectra of dansyl-AgNPs (1 mg mL−1) in the presence of Cu2+ and various other metal ions (0.5 μM). (D) Fluorescence spectra of dansyl-AgNPs (1 mg mL−1) upon addition of Cu2+ from 0.5 nM to 0.5 μM

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Some recent reports for fluorescence method-based sensing systems are listed in Table 4.2.

Table 4.2 List of fluorescence sensors based on nanomaterials
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4.6.3 Raman/Surface-Enhanced Raman Spectroscopy (SERS) Sensors

Raman spectroscopy is one of the important analytical techniques used in the field of sensors for identification of specific molecules. But traditional Raman spectroscopy has very poor efficiency of inelastic scattering processes with relatively weak signal. In recent years these problems are overcome by developing advanced method like surface-enhanced Raman spectroscopy (SERS). In SERS, the target molecule is brought into close proximity to a metallic (typically Ag, Au, or Cu) surface with nanoscopically defined features or in solution next to a nanoparticle with a diameter much smaller than the wavelength of the excitation light.

Silver nanoparticle decorated on filter paper as a highly sensitive surface-enhanced Raman scattering (SERS) substrate was fabricated for detection of tyrosine in aqueous solution (Cheng et al. 2011) with the detection limit as low as 625 nM. The Raman labels to use for detection of protein and protein concentration assay were developed (Han et al. 2010) which uses the SERS signal of Coomassie Brilliant Blue dye adsorbed nonspecifically to silver colloids for monitoring the total protein concentration.

Silver nanoparticles decorated on a cylindrical support was developed (Rajapandiyan et al. 2011) for the detection of melamine in milk liquid and powder samples by surface-enhanced Raman spectroscopy (SERS) technique. Figure 4.6A illustrates the detection procedures in the cylindrical SERS substrate. Cylindrical SERS substrate was prepared by decoration of silver nanoparticles (AgNPs) on a solid support of polymethyl methacrylate (PMMA) rod by silver mirror reaction. SEM images show the PMMA surface roughness was increased after treating with PVDF solution. Also, the size of the formed AgNPs was around 200 nm in diameter with the Raman spectra of solid melamine and SERS spectra of melamine as shown in Fig. 4.6B. Sensitivity response of AgNP cylindrical substrate to the melamine concentration ranges was tested up to 125 ppm, and their corresponding SERS spectra are shown in Fig. 4.6C.

Fig. 4.6
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(A) Schematic diagram of SERS detection of target molecules using droplet-based method. (B) SEM images of bare PMMA rod (a), PVDF-coated rod (b), and AgNPs decorated on PVDF-coated rod (c). Raman spectra of solid melamine and SERS spectra of melamine in water (400 ppb) detected by cylindrical substrate (d). (C) SERS spectra of melamine in milk liquid at 0, 5, 15, 50 and 125 ppm

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Some recent reports for SERS method-based sensing systems are listed in Table 4.3.

Table 4.3 List of SERS sensors based on nanomaterials
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4.6.4 Electrochemical-Based Sensors

Electrochemical methods have been used extremely in analysis of biological and environmental interest compounds due to their advantages such as quick response, wide linear dynamic range, simplicity, reliability, reproducibility, inherent miniaturization, low cost, low-power requirements, and high selectivity and sensitivity. The type of electrical signal used for quantitation distinguishes the various electroanalytical techniques. For example, in case of voltammetry, the current is measured as a function of changing potential that can be applied in different ways (linear, cyclic, anodic stripping voltammetry). Amperometric measurements are performed by maintaining a constant potential at the working electrode with respect to reference electrode and measuring the generated current. The generated potentials or currents are related to the contents of analyte in the test solution (Zoski 2007). The researchers first tried to detect compounds using carbon and metal electrodes without any chemical modification. But the problems such as oxidation or reduction occur at high over potential, suffer from interferences and kinetically sluggish. Therefore, the development of electrochemical sensor based on chemically modified electrodes for estimating biologically important compounds is a rapidly growing area of electrochemistry to overcome the problems associated with bare electrodes. Particularly, the chemically modified electrodes have been fabricated using nanomaterials such as metal nanoparticles , metal oxide nanomaterials, and carbon nanomaterials .

Silver nanoparticles incorporated within the mesoporous silicate framework of zeolite Y-modified glassy carbon electrode-based electrochemical sensor were developed for the simultaneous detection of dopamine (DA) and uric acid (UA) (Meenakshi et al. 2016). The oxidation of DA and UA was obtained at +0.31 V and +0.43 V (vs. Ag/AgCl) using AgNPs/Zeo-Y/GCE under the optimum experimental condition. A well-resolved peak potential window (~120 mV) for the oxidation of both DA and UA was observed at AgNPs/Zeo-Y/GCE system. This is due to the strong electrostatic interaction between the positively charged DA and negatively charged silver nanoparticles embedded in zeolite Y which facilitates the electron transfer process is favorable and the oxidation peak potential of DA is resolved against UA. The detection limits of DA and UA were found to be 1.6 × 10−8 M and 2.51 × 10−8 M in the linear range of 0.02 × 10−6 to 0.18 × 10−6 M (R2 = 0.9899) and 0.05 × 10−6 to 0.7 × 10−6 M (R2 = 0.9996) by using differential pulse voltammetry (DPV) method.

The electrochemical oxidation of paracetamol (PAR) and caffeine (CAF) using nickel hexacyanoferrate-decorated titanium nanotube (TNT)-modified glassy carbon electrode was developed (Fig. 4.7A) (Devi and Pandian 2014). The SEM image of TNT shows the formation of tube-like structure with the external diameter of 10–80 nm ranges and several micrometers in length. The TEM image shows that the TNT wall consists of two layers with an average external diameter of 28 nm. After surface modification with NiHCF, the size and shape of TNT become expanded with the uniform deposition of NiHCF on both inner and outer surfaces (Fig. 4.7B). DPV experiment was carried out for the simultaneous detection of PAR and CAF with well-separated peak potentials. The determination of PAR has been done separately in the concentration ranges of 1.3–10.7 μM with the solution containing 6 μM CAF (Fig. 4.7C). Similarly, the determination of CAF has been studied in the concentration ranges of 7.3–18.7 μM in presence of PAR at a fixed concentration of 1.3 μM (Fig. 4.7D). The detection limits of PAR and CAF are found to be 29.5 and 18.2 nM, respectively. Some recent reports for electrochemical method-based sensing systems are listed in Table 4.4.

Fig. 4.7
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(A) Mechanisms for the oxidation of paracetamol and caffeine at NiHCF/TNT@GCE. (B) FE-SEM and HR-TEM images of TNT (a and c) and NiHCF/TNT (b and d). (C) DPV of CS-NiHCF@TNT/GCE at concentration of PAR (a–h, 1.3–10.7 μM) and 6 μM of CAF. (D) DPV of CS-NiHCF@TNT/GCE at concentration of CAF (a–i, 7.3–18.7 μM) and 1.3 μM of PAR. The inset shows the calibration plot

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Table 4.4 List of electrochemical sensors based on nanomaterials
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4.7 Conclusions

Remarkable progress in the development of simple and efficient methods for sensing of chemical and biomolecules utilizing nanomaterials has been made over the past several years. The successful development of sensors with high selectivity and sensitivity for specific, toxic, heavy/biologically essential metal ions and biomolecules to detect in real-time monitoring was clearly demonstrated. In this chapter, we believed that most promising results and remarkable sensing system in terms of high selectivity and sensitivity have been discussed. However, many of the absorption- and fluorescence -based sensing systems were operated in aqueous medium, and some other system using nonaqueous solvents was applied for real sample analysis. Fluorescence sensors based on surface-functionalized nanomaterials have been widely used in biological and environmental sample analysis, due to their high sensitivity, low cost, and commercial availability. SERS technique has great promise for chemical and biosensing applications based on design of paper, and also fiber-based SERS active substrates have very high surface area with flexibility for efficient analysis of biological live samples. Electrochemical techniques are coupled with nanomaterials that have been used for the development of electrochemical sensors . The attractive design and fabrication of nanomaterial-based electrochemical sensing systems can offer a highly selective increase in sensitivity and reproducibility. Most of the sensing systems with a very low detection limits (in the range of ppb) were thus achieved and applied for the real sample analysis.