Introduction

Nitrobenzene (NB) is a widely used solvent in manufacturing processes and is often discharged by industries as a waste. Increased industrial development and increased discharge of waste is resulting in surface and ground water pollution. NB is a high priority pollutant in the nitroaromatics (NACs) declared by the Environment Protection Agency on the basis of its known carcinogenicity, mutagenicity, and acute toxicity [1, 2]. Conventional analytical methods used for the detection of NB include liquid–liquid extraction–gas chromatography, solid phase micro-extraction, nuclear quadrupole resonance, and electron capture techniques [3–7]. These techniques are highly selective and sensitive but are very expensive, laborious, and time consuming. Nanomaterials of size few to 100 nm have enabled new types of sensors that are capable of detecting extremely small amounts of analytes in lower limit range [8, 9]. However, nanoparticles (NPs) suffer from the aggregation problems, which revert them back to the bulk materials. To prevent agglomeration, metal NPs are often coated with ligands, polymers, organic surfactants, mesoporous/nanoporous supports, which not only delimit the particle size but also help in immobilization of the resulting NPs [10–12]. Owing to the large internal surface area and small pore size, mesoporous material finds applications in catalysis, chromatography supports, optics, photonics, semiconductor devices, and chemical sensors [13]. In the present work, polyvinylpyrollidone (PVP)-coated gold nanoparticles (Au NPs) have been synthesized and then functionalized with silica for NB detection. The formation of PVP and the silica layer on the particle surface helps in increasing the aggregation stability of the NPs by decreasing the inter phase tension via strengthening interactions between the dispersed phase and the dispersion medium. This in turn increases the entropy component of the system due to the involvement of molecules and ions of the surface layer in thermal motion together with particles of the dispersed phase [14]. Additionally, porous silica helps in the adsorption of analytes. Silica minerals are reported as one of the most efficient adsorbents for NACs contaminants [15].

Herein, we are reporting SiO2/Au NPs as a dynamic range sensor towards NB.

Experimental details

Materials

All chemicals were of analytical grade and used as received without further purification: HAuCl4 (Spectrochem Pvt. Ltd., Mumbai, India), PVP (Sisco Research Lab, Mumbai, India), tetraethyl orthosilicate (TEOS, Merk Specialties Pvt. Ltd., Darmstadt, Germany), ammonium hydroxide (S.D. Fine Chem. Ltd., Mumbai, India), ethanol (Changshu Yangyuan Chemical China, Changshu, China), ethylene glycol (Loba Chemie, Mumbai, India), and NB (Spectrochem Pvt. Ltd.). De-ionized water obtained from Millipore was used for all synthesis and experimental studies.

Preparation of SiO2/Au NPs

Au NPs were synthesized using hydrazine hydrate reducing agent and PVP capping agent. Briefly, PVP (1 g) was added into distilled water (50 ml) under continuous stirring with following additions of 1 % HAuCl4 solution (5 ml). To this mixture, hydrazine hydrate (1 ml) was added and stirred until the appearance of wine red color which confirms the formation of Au NPs. Synthesized Au NPs were functionalized by silica using Lu et al. approach [11]. Briefly, Au NPs colloidal solution (4 ml) was added to ethanol (20 ml) under constant stirring followed by addition of ammonium hydroxide and TEOS (5–10 μl). This solution was stirred at room temperature for about 1 h and then centrifuged at 8,000 rpm for 30 min to collect coated NPs. Figure 1 shows the pictorial presentation of steps used in the synthesis and silica coating of Au NPs.

Fig. 1
figure 1

Schematic for synthesis of SiO2/Au NPs

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For electrochemical sensing, NB stock solution was prepared in acetonitrile and NaCl (0.1 M) was used as an electrolytic medium. Varying amounts (1–300 μl) of NB (0.01 mM) was added to the electrolyte solution and mixed properly prior to each voltammetric measurement.

Instrumentation

Optical studies were done using PerkinElmer® Lamda 35 UV–visible spectrophotometer. Scanning electron microscope (SEM) and energy-dispersive x-ray analysis (EDXA) analysis was carried out on FE-SEM (Oxford Company) equipped with an X-ray analyzer for morphological and elemental information. Glassy carbon (GC) electrode spin coated with SiO2/Au NPs was used as working electrode for voltammetric and impedance studies in 0.1 M K3[Fe(CN)6]/KCl solution (used as a redox probe) using the CHI-660 Instrument.

Results and discussion

Characterization of SiO2/Au NPs

Figure 2 shows EDXA spectrum of SiO2/Au NPs indicating presence of both Au (AuM, 2.2; AuL, 10.0 keV) and silica (SiK, 1.5–1.8 keV) along with a prominent peak for copper (CuK, 8.2 KeV; CuL, 0.9 KeV) which is possibly due to X-ray emission from the copper substrate. EDXA spectra were collected over random areas to monitor the homogeneity of elemental composition. The presence of Au and silica peaks is consistent with the formation of silica shell on Au NPs. Inset shows the transmission electron microscopy (TEM) image of SiO2/Au NPs at high resolution confirming coating of Au NPs with silica. It can be clearly seen that the size of Au NP is around ∼38 nm and shell is of thickness ∼18 nm.

Fig. 2
figure 2

EDXA spectrum of SiO2/Au NPs (inset: (a) SEM image of SiO2/Au NPs corresponding to EDXA spectrum and (b) TEM image of SiO2/Au NP at higher resolution)

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Figure 3 shows UV–visible spectrum of Au (a) and SiO2/Au NPs (b and c). The surface plasmon resonance (SPR) peak position at 528 nm confirms the formation of spherical Au nanoparticles [12]. A shift of ∼3 nm was observed on coating synthesized Au NPs with silica and is attributed to the change in the refractive index of the surrounding medium from 1.36 (ethanol) to 1.45 (silica) [16]. On further increasing TEOS amount (5 to 10 μl) for silica coating, no change in SPR peak position is observed, however, the intensity of SPR band increased (Fig. 3). This might be because of the increased scattering from thicker shells. The broadening of the SPR band after coating can be attributed to the roughness of the shell surface or the presence of a few particles with incomplete shells as earlier reported by Lu et al. [13, 17].

Fig. 3
figure 3

UV–visible spectra of (a) Au NPs and (b, c) SiO2/Au NPs synthesized with varying TEOS amount, 5 μl and 10 μl, respectively

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Characterization of SiO2/Au NPs-modified electrode

Impedance and voltammetric studies

Figure 4 shows interfacial features of bare (GC) and modified (GC/SiO2/Au NPs) electrodes in 0.1 M K3[Fe(CN)6]/KCl solution. Nyquist plot (real (Z ′) vs. imaginary parts (Z ″) of the impedance) of SiO2/Au NPs-modified electrode (GC/SiO2/Au NPs) shows somewhat flattened semicircle at higher frequencies and a straight line forming an angle of 45° to the real axis at lower frequencies. The flattened circle is a consequence of roughness due to the material (SiO2/Au NPs) deposited on the electrode surface. A perfect semicircle corresponds to perfectly smooth surface and this circularity decreases with an increase in surface roughness [18, 19]. The straight line at lower frequencies can be attributed to the Warburg impedance which becomes dominant at lower frequencies for diffusion limited processes. Its appearance for modified electrode (GC/SiO2/Au NPs) may be due to the presence of insulating silica coating around Au NPs, which may partially limit the transport of ions to the electrode surface as indicated by increase in charge transfer resistance (R ct) of the bare electrode from 1.88 × 102 to 2.51 × 102 after modification. This increase in R ct value is possibly due to the steric hindrance and electrostatic repulsion between surface SiO −2 groups and negatively charged redox couple [20, 21].

Fig. 4
figure 4

Nyquist Plot of (a) bare GC and (b) SiO2/Au NPs-modified GC electrode in 0.1 M [K4Fe(CN)6 + KCl] solution

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Figure 5 shows cyclic voltamograms of bare (GC) and modified (GC/SiO2/Au NPs) electrodes in 0.1 M K4(Fe(CN)6) + KCl solution. Cyclic voltammogram (CV) of modified electrode in ferricyanide solution is a valuable tool to monitor the barrier of the modified electrode since the electron transfer between the solution species and the electrode must occur either through the barrier itself by tunneling or through the defects in the material/barrier. It is well proved that when the bare electrode surface is modified by some materials, the electron-transfer kinetics of Fe (CN) 4−/3−6 is perturbed.

Fig. 5
figure 5

CV of (a) bare GC and (b) SiO2/Au NPs-modified GC electrode in 0.1 M [K4Fe(CN)6 + KCl] solution as a function of scan rate (40 to 180 mV/s), inset: an overlay of bare GC (blue curve) and Si@Au NPs/GC (red curve) response in K4Fe(CN)6 electrochemical solution

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It can be clearly seen from Fig. 5a, b that the CV response of the bare GC electrode shows only one oxidation peak (Fe+2 to Fe+3) whereas modified (GC/SiO2/Au NPs) electrode shows the presence of a proper redox couple for Fe2+/Fe3+ ions with an enhanced current sensitivity (102 μA/mM) relative to the bare GC (inset of Fig. 5b). This appearance of a redox couple and increase in the current signal for modified electrode could be explained by the increase in the effective electrode surface area because of modification with NPs [22]. Peak current (I P) increases linearly with the square root of scan rate which indicates the dominance of diffusion in this electrochemical process. This behavior can be explained by Randles Sevick equation [23].

$$ {i_p} = 2.69 \times {10^5}{n^{{ \frac{3}{2} }}}A{D^{{ \frac{1}{2} }}}C{v^{{ \frac{1}{2} }}} $$
(1)

From the above equation, the diffusion coefficient (D) for modified electrode is found to be 1.93 × 10−8 cm2/s.

Detection of NB

Optical detection

The optical detection of NB using SiO2/Au NPs is based on the change in the intensity of the SPR band of NPs after incubation with NB and is basically based on adsorption chemistry. Because of high mesoporosity and surface roughness resulting in a large surface-to-volume ratio, SiO2/Au NPs offer a vast surface area for efficient interaction with the analyte. In the presence of NB, intensity of the SPR band of SiO2/Au NPs decreases with no effect on band position (Fig. 6).

Fig. 6
figure 6

UV–visible spectra of SiO2/Au NPs in the absence (blue curve) and presence of (a) 10−6 M, (b) 10−5 M, (c) 10−4 M, (d) 10−3 M, (e) 10−2 M, (f) 10−1 M NB

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In our case, this sensing might be due to the interaction between –NO2 groups of NB and surface hydroxyls present on the silica surface. This interaction results in the formation of Meisenheimer complex which is a σ-complex formed by covalent addition of nucleophile to an arene carrying electron deficient aromatic compound. Since NB is electron deficient due to the strong electron withdrawing effect of NO2 group, NB is able to form Meisenheimer complex. From available literature, it is evident that such an interaction has been employed for designing the surface chemistry of nanostructures and electrodes to achieve the selectivity and sensitivity for the detection of TNT using fluorescence and electrochemical techniques [24–26]. This complex formation is believed to cause the damping of surface plasmon resonance due to gold core. The exact mechanism by which this occurs is not known, but the literature suggests the possibility of an increase in the imaginary part of the dielectric constant of gold [27–29]. The adsorption of NB introduces a thin layer of with the modified electron density on gold cores which produces a damping effect on SPR.

The sensing ability of SiO2/Au NPs towards NB has been determined as a function of NB concentration as shown in Fig. 6. As the concentration of NB increased from 10−6 to 10−1 M, intensity of SPR of SiO2/Au NPs at 530 nm decreased linearly (inset, Fig. 6).

Electrochemical detection

It is evident from the literature that metallic NPs can catalyze oxidation/reduction of various organic compounds [30] but they suffer the aggregation problem. To explore the capability of SiO2/Au NPs to enhance the sensitivity of the electrode towards NB via surface absorption by silica and reduction at Au cores [30–32], CV studies were performed by incubating SiO2/Au NPs-modified electrode in 0.5 M NaCl electrolyte solution containing different amounts of 0.01 mM NB.

In the absence of NB, no reduction and oxidation peaks were observed for SiO2/Au NPs-modified electrode (Fig. 7, inset, curve c) incubated in 0.1 M NaCl. With the addition of NB in electrolytic solution, two cathodic reduction peaks, C 1 (−0.74 V) and C 2 (−0.45), and two anodic oxidation peaks, A 1 (−0.62 V) and A 2 (0.03 V), were observed as shown in Fig. 7 (inset, curve d). A 1/C 1 can be attributed to the four-electron irreversible reduction of the nitro group (–NO2) to the hydroxylamine derivative (–NHOH) and A 2/C 2 is assigned to the two-electron reversible oxidation of the hydroxylamine group (–NHOH) to a nitroso group (–NO) as shown below in Eqs. 2 and 3 [33, 34].

$$ - {\text{N}}{{\text{O}}_2} + 4{e^{ - }} + 4{{\text{H}}^{ + }} \to - {\text{NHOH}} + {{\text{H}}_{{2}}}{\text{O}} $$
(2)
$$ - {\text{NHOH}} \leftrightarrow - {\text{NO}} + 2{{\text{H}}^{ + }} + 2{e^{ - }} $$
(3)
Fig. 7
figure 7

CV of (a) bare GC, (b) SiO2/Au NPs/GC electrode in 0.1 M NaCl solution containing 0.01 mM NB, inset: CV of (c) SiO2/Au NPs/GC NaCl without NB and (d) with NB (0.01 mM)

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A significant enhancement (∼30 %) in reduction peak current (C 1) is observed at modified electrode (GC/SiO2/Au NPs) relative to that at bare electrode (Fig. 7, curves a and b) and is attributed to the synergic effect of surface –OH groups selectivity of mesoporous silica towards NB and its subsequent catalytic reduction by metallic Au cores [31, 32].

Figure 8 shows the CV response of the modified electrode as a function of NB concentration (1 × 10−4 to 2.5 × 10−2 mM). A monotonic increase in peak current with an increase of NB concentration is observed. The detection limit calculated from linear response curve (Fig. 8 (b)) is found to be 12.3 ppb and the current sensitivity is ∼102 μA/mM (R 2 = 0.985). It can be seen in Fig. 8 (b) that the curve is having linearity up to 2.3 × 10−2 mM NB concentration and a kink after that. The level at which the kink is observed may be assigned as the threshold point of the sensing platform as the graph is curvilinear after this value.

Fig. 8
figure 8

(a) CV of SiO2/Au NPs/GC electrode in 0.1 M NaCl solution contacting varying amount of NB (10 to 300 μl), inset: for 1 to 10 μl of NB and (b) a relation curve between reduction peak current, C 1, and NB concentration (1 × 10−4 to 2.5 × 10−2 mM)

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Interference studies with ions NO −3 , Ni2+, and Zn2+ and phenol (0.1 M) were done under the same experimental conditions and no interference of these species on sensor performance was observed. The selectivity of this sensing platform towards specific nitro-compound can be further introduced by modifying the silica surface with specific organic moieties [35]. The effect of other parameters such as film thickness, pH, and temperature on sensitivity is under study.

Conclusion

Silica-coated Au NPs (SiO2/Au NPs) have been explored as a wide-range sensitive sensing platform towards NB. Acid–base chemistry (Meisenheimer complex) between hydroxyl groups presented on the mesoporus silica surface and electron-deficient aromatic ring of NB was used for NB immobilization and subsequent catalytic reduction by metal (Au) core. Damping in SPR intensity of Au NPs was observed on NB immobilization, which may be explained by an increase in the imaginary part of the dielectric constant on NB adsorption. The optical detection range is observed in window 10−1 to 10−6 M while electrochemical detection range is found to be in the narrow window (10−4 to 2.5 × 10−2 mM) with a detection limit of 12.3 ppb and sensitivity of 102 μA/mM relative to bare and silica-modified electrode.