Dimethylenebis-(tetra-decyldimethylammonium Bromide)-Driven Metal Nanoparticles: Hg2+ Sensing a Competency.

We report an excellent anisotropic Au nanoparticle-based colorimetric probe for the detection of Hg2+ ions with higher detection ability and selectivity. The manifestation of different morphologies of Au nanoparticles including round, triangular, rectangular, pentagonal, and hexagonal has been realized by the dimethylenebis-(tetra-decyldimethylammonium bromide) (14-2-14 Gemini surfactant) assisted one-step thermal reduction method where the average size of Au nanoparticles was 54.65 ± 44.3 nm. The growth and frequency of Au nanoparticles were enhanced as a function of Gemini surfactant's concentration. The detection limit as low as 1.8 nM was efficaciously achieved and was considerably lower than the required world standards defined the maximum allowable level of Hg2+ ions for health hazards. Notably, the Au nanoparticles showed visible detection for 100 μM Hg2+ ion by means of the change in the solution color from red to tarnish blue within 180 s followed by saturation in the absorption ratio (A LSPR/A TSPR). These results provide novel insight into the detection of the heavy metal ion using Gemini surfactant-assisted grown anisotropic metal nanoparticles. On the basis of obtained results, it is concluded that the size of metal nanoparticles is no longer critical for preparation of efficient selective chemoprobe; rather, growth of more number of edges provides a large number of sights for incoming moieties and plays an important role in improving the detection capability of the anisotropic metal nanoparticle irrespective of their large sizes. We believe that this work provides valuable insight into researchers working in the area of chemosensor applications.

Introduction

Heavy metal ions can cause severe threats to the environment and human health, and the potential sources of substantial metal ions are mining, agriculture, toxic wastes, electronic merchandise, and natural discharge from earth’s crust into the surroundings. This discharge from oceanic and volcanic emissions end up in the contamination of underground water resources and human beings get exposed to the same. Among aforesaid ions, mercury ions are incredibly fatal and may cause serious health problems if consumed by individuals, leading to numerous critical diseases such as harmful effects on immune systems, cancer, organ damage, nervous system damage, etc.[1,2] According to the World Health Organization (WHO) and U.S. environmental protection agency, the detection limit for Hg2+ are 30 and 10 nM, respectively;[3,4] therefore, it is essential to achieve standards via reliable ways or procedures.

Nanomaterial-supported colorimetric approaches have attracted excessive attention due to their distinctive advantages such as ease of fabrication, fast monitoring, and even detection with an eye.[5] Normally, the fluorescence method,[6] surface-enhanced Raman spectroscopy,[7] surface plasmon resonance (SPR) approach,[8] and conventional methods such as atomic absorption spectroscopy,[9] inductively coupled plasma mass spectrometry,[10] and reversed-phase high-performance liquid chromatography[11] have widely been used for a precise qualitative analysis. Among these, SPR analysis encompasses a candid sample preparation, consistent and needs low-cost tools compared to aforesaid other techniques. The SPR-based colorimetric analysis acknowledges detection of heavy metal ions by a noticeable shift in SPR resulting via aggregation of metal nanoparticles followed by a change in the solution color.

A variety of nanomaterials have been established to be effective for mercury ion monitoring including noble metal nanoparticles (Au and Ag), semiconductor nanocrystals, polymeric materials, biomaterials, etc.[12,13] Among them, metal nanoparticles are found promising for selective detection of Hg2+ ions.[14] The detection of heavy metal ions immensely rely on the shape and size of metal nanoparticles and accessibility of functionalized species on their surface, which either directly interacts with heavy metal ions or reduces them to interact with metal nanoparticles.[15,16] Notably, metal nanoparticles of sizes 5 to 15 nm have been found promising for the detection of heavy metal ions; however, it requires a multistep process, usage of harsh reducing agents, and vast quantitative control over chemical reagents.

The anisotropic metal nanoparticles have received considerable attention attributed to the distinguished optical properties and adaptable surface modification viability. The preferential growth of low-energy facets provides active absorption sites for interfacial interaction between the metal nanoparticle and heavy metal ions, thereby augmenting sensitivity regardless of their large size (greater than 100 nm).[17,18] Therefore, use of these nanostructures may be a choice to improve the detection of heavy metal ions. Foremost, the seed growth method is the well-established chemical route for synthesizing controllable anisotropic nanoparticles.[19] However, vigilant processing entails higher quality seed solution with a uniform size generally smaller than 5 nm. The method may involve multiple steps depending upon the requirement of shape and size of particles at the nanoscale. In addition, a higher quantity of conventional surfactants required due to their higher critical micelle concentration (cmc). Inevitably, an easy and reliable approach is required to overcome abovementioned issues.

Gemini surfactant (GS) is a class of surfactant with two hydrocarbon tails and two polar head groups covalently joined by a spacer compare to the single hydrocarbon tail and polar head group in conventional surfactants. These features effectively enhance their absorption ability at the liquid–solid interface even at low cmc.[20] Moreover, the hydrocarbon chain length considerably affects the size and shape of metal nanoparticles.[21,22] According to our recent investigation,[22−24] GSs of varied hydrocarbon chain length and concentration significantly influenced the shape and size of the nanomaterials and concluded that the GS acts as a shape-directing agent; it helps to grow nanoparticles at the nanoscale with highly unstable edge or corner regions, which is indeed highly desirable for chemosensor applications. The enhancement in the chain length of GS (m = 16, 18, and 20) resulted in a shape transformation from triangular particles with truncated corners to smooth round corners of anisotropic growth of Au nanoparticles. The changes in the particle shape and size were found critical toward detection of Hg2+. The large-sized triangular nanoparticles prepared with a smaller chain length showed the best sensing ability, whereas the detection capability dramatically lowers for Au nanoparticles synthesized with a larger chain length.[22] This clearly indicates that the reduction in the hydrocarbon chain length can influence the sensing ability of metal nanoparticles by means of controlling the growth and promoting the degree of anisotropy.

In order to confirm these, it is essential to investigate the influence of the shorter hydrocarbon chain length on the detection performance of metal nanoparticles. In view of that, we selected dimethylenebis-(tetra-decyldimethylammonium bromide) (14–2-14) GS with a shorter hydrocarbon chain length and further investigated sensitivity and selectivity of anisotropic nanoparticles. The well-defined morphologies of Au nanoparticles were prepared by the one-step approach and were able to detect Hg2+ as low as 1.8 nM with high selectivity, which is the best among Au nanoparticle-based chemosensor probes prepared using GS of having different hydrocarbon chain lengths.

Results and Discussion

Figure demonstrates UV–visible absorption spectra of the Au nanoparticles synthesized by varying the concentrations of (14-2-14) GS. All the spectra illustrate the two surface plasmon resonances (SPR) confirming the formation of Au nanoparticles irrespective of the change in GS concentrations. The SPR absorbance appears because of the resonance between conduction electrons and incident electromagnetic radiations. The transverse surface plasmon resonance (TSPR) crests have been observed as a major peak around 536–542 nm and a weak longitudinal surface plasmon resonance (LSPR) peak around 718–727 nm. The presence of two SPR absorbance bands indicates that the nanoparticles have anisotropic morphology. The trend clearly displays red-shift (in the case of TSPR is ∼6 nm and in the case of LSPR is ∼9 nm) in a consistent manner as the concentration of GS increased from 0.2 to 3 mM suggesting that Au nanoparticle size increased as a function of surfactant concentration. Notably, no LSPR is observed for Au nanoparticles synthesized without (14-2-14) GS, and only a TSPR peak was observed at 530 nm, which generally indicates the formation of monodispersed, round-shaped Au nanoparticles. The corresponding UV spectrum of Au nanoparticles is not included to avoid any redundancy. The appearance of red-shifts in the absorbance of both TSPR and LSPR peaks suggests that the (14-2-14) GS serves as a shape-regulating mediator for Au nanoparticles where the shapes and sizes of Au nanoparticles are extremely reliant on the concentration of the surfactant.

Figure 1

UV–vis absorption spectra of Au nanoparticles at different concentrations (0.2–3 mM) of (14-2-14) GS.

For further investigation of the impact of the (14-2-14) GS concentrations on the shape and size of Au nanoparticles, the microstructural analysis was performed by transmission electron microscopy (TEM). The three concentrations, 0.2, 2, and 3 mM, were selected as they have shown a maximum shift in both TSPR and LSPR. Figure a–c shows transmission electron micrographs of Au nanoparticles synthesized at 0.2 mM (14-2-14) GS. The same figures divulged that diverse-shaped Au nanoparticles including ground, triangular, rectangular, pentagonal, and hexagonal were prepared, and among all of them, the number of round shapes was more than the other shapes. The number of Au nanoparticles of different morphologies is summarized in Figure d. Figure e,f demonstrates high-resolution TEM images of Au nanoparticles of hexagonal shape where the surface of the nanoparticle was covered by a very thin layer of thickness ∼1.1 nm, which may be a mixture of (14-2-14) GS and sodium citrate discussed in detail in the text at the later stage.

Figure 2

(a–c) Transmission electron micrographs of Au nanoparticles at 0.20 mM (14-2-14) GS. (d) Histograms of particle shape distribution. (e) Magnified TEM image of hexagonal-shaped Au nanoparticles. (f) High-magnification image of a single particle capped with a surfactant layer of thickness ∼1.1 nm.

Figure a–d displays TEM images of Au nanoparticles synthesized with 2 mM concentration of (14-2-14) GS. Evidently, the numbers of differently shaped nanoparticles are enhanced at the higher concentration of (14-2-14) GS except for round- and hexagonal-shaped nanoparticles. The particle distribution in relation to their shape is compiled in Figure e. The numbers of triangular, pentagonal, and rectangular nanoparticles increased by 12, 11, and 2%, respectively, compared to the Au nanoparticles synthesized with 0.2 mM concentration of the (14-2-14) GS, whereas the number of round- and hexagonal-shaped nanoparticles was decreased by 21 and 4%, respectively. It suggests that the GS significantly promotes the anisotropic growth of Au nanoparticles while consuming round- and hexagonal-shaped nanoparticles. Indeed, the change in the number of nanoparticles in relation to the concentration of (14-2-14) GS indicates that the growth of nanoparticles occurs on the expense of round-shaped nanoparticles where the geometrical axis of the nanoparticle is eaten up during the growth process in sequence from round- to hexagonal- to pentagonal- to triangular-shaped nanoparticles. For better understanding, the high-resolution TEM image having different-shaped nanoparticles is shown in Figure f–h. The HRTEM image in Figure g taken from Figure f shows a Au nanoparticle attained an intermediate geometrical transformation state during the growth process where the shape and size of the nanoparticle are changing from one shape to the other. Here, the shape of these Au nanoparticles is referred as round. It is also supported by the fact that the number of pentagonal and triangular nanoparticles is increased remarkably in relation to the decrease in number of round- and hexagonal-shaped nanoparticles. Figure h showing a high-resolution TEM image evidently confirmed that the particle has a fine pentagonal shape, and their surface was covered by the thin layer of thickness ∼1.2 nm, which is a very much comparable thickness present on the surface of Au nanoparticles synthesized using 0.2 mM surfactant. Figure i shows the EDX elemental analysis specifying the manifestation of the presence of only gold. Here, an extra peak for Cu appears from the copper grid used for the analysis.

Figure 3

(a–d) TEM images of Au nanoparticles at 2 mM (14-2-14) GS and (e) histograms of particle shape distribution. (f) Magnified TEM image of Au nanoparticles. (g) High-magnification image of a single particle capped with a surfactant layer of thickness ∼1.2 nm and (h) EDX spectrum of Au nanoparticles at 2 mM (14-2-14) GS.

Figure a–e shows TEM images of Au nanoparticles synthesized with 3 mM concentration of (14-2-14) GS. As is clear, no significant change in numbers of differently shaped nanoparticles was observed at the higher concentration of (14-2-14) GS. The particle distribution in relation to their shape is compiled in Figure f. The number of round-shaped nanoparticles increases significantly, and other shapes including hexagonal, pentagonal, and tetragonal are nearly similar or decreased significantly compare to 2 mM (14-2-14) GS. This clearly indicated that the higher concentration of GS hinders the growth of Au nanoparticles and hence suppressing the longitudinal surface plasmon resonance. For better understanding, the changes in particle number distribution in relation to GS concentration are summarized in Figure .

Figure 4

(a–e) TEM images of Au nanoparticles at 3 mM (14-2-14) GS. (f) Histograms of particle shape distribution.

Figure 5

Histogram of Au nanoparticle shape distribution synthesized using 0.2, 2, and 3 mM.

Along with the Au nanoparticle shapes, the size was also influenced by the surfactant concentration. Figure S1a–c shows the bar graph of the particle size distribution of Au nanoparticles. The average size of Au nanoparticles was increased from 41.54 ± 32.0 nm to 54.65 ± 34.3 nm and further decrease to 35.97 ± 25.30 nm as the concentration of GS increased. For a vibrant insight, the average sizes of nanoparticles with various shapes are outlined in Table . The round-shaped nanoparticles are almost similar in size, while the size of other shapes is higher for 2 mM GS concentration compare to the other concentration of the surfactant. The change in the particle number and size of Au nanoparticles suggests that the (14-2-14) GS not only influences the growth of metal nanoparticles but also acts as a shape-directing agent.

Table 1

Particle Size Distributions of Au Nanoparticles Synthesized with (14-2-14) GS

			average size of particles with different shapes (nm)
Gemini surfactant	concentration (mM)	average particle size (nm)	round	triangular	rectangular	pentagonal	hexagonal
(14-2-14)	0.2	41.54 ± 32.0	36.8 ± 19.3	50 ± 20.6	45.0	47.5 ± 12.2	33.3 ± 10.4
	2	54.65 ± 44.33	34.4 ± 19.2	57.8 ± 33.1	87.5 ± 24.7	56.6 ± 22.5	41.0 ± 14.3
	3	35.97 ± 25.30	35.2 ± 22.2	49.2 ± 24.2	84.7 ± 10.3	48.6 ± 30.5	32.8 ± 11.3

In an attempt to enumerate the surface adsorption of the surfactant on the Au surface, Figure S2 illustrates comparative FTIR spectral studies of pure (14-2-14) GS and Au nanoparticles synthesized in the presence of (14-2-14) GS with the same concentration of the surfactant. The FTIR of pure sodium citrate is also included for the sake of comparison. Also, the peak assignments in relation to the (14-2-14) GS are summarized in Table S1. The FTIR result endorses the presence of GS on Au nanoparticles and supports the results obtained by TEM analysis.[26−32]

Further, Au nanoparticles prepared with the 2 mM concentration of (14-2-14) GS were used as a colorimetric assay for the detection of Hg2+ ions as it consists of more number of diverse-shaped Au nanoparticles. The stable Au nanoparticle dispersion exhibits electrostatic repulsion between nanoparticles and maintains stability via restricting aggregation, thereby suggesting its viability for obtaining superior sensing. In order to obtain improved sensitivity and detection limit of the colorimetric assay, the salinity conditions (NaCl and pH) were optimized because it overcomes the electrostatic repulsion between Au nanoparticles and offers low barrier for the metal ion-induced nanoparticle aggregation. The freshly prepared different concentrations (0.10–200 μM) of Hg2+ ions were mixed with the aforementioned colorimetric assay and allowed to equilibrate for 5 min. Subsequently, the prepared solutions were transferred into a quartz cuvette for recording UV–visible spectral data. Figure a shows the UV–vis spectra of Au nanoparticles with different Hg2+ ion concentrations. The Au nanoparticles gradually aggregated with an increase in Hg2+ concentration. Accordingly, UV–visible spectra of Au nanoparticles also show a noticeable TSPR shift from 542 to 572 nm shown in Figure b, whereas an upward shift in LSPR was also observed, which suggests that the change in the average size is due to the agglomeration of Au nanoparticles. In order to investigate it, the TEM analysis of Au nanoparticles collected after detection of Hg2+ was performed, as shown in Figure S3. As is clear, the Au nanoparticles were agglomerated after the addition of Hg2+, and no change in morphology of anisotropic Au nanoparticles occurred after detection of Hg2+. The gradual change in the Au nanoparticle solution color from red to tarnish blue by the addition of Hg2+ is shown in Figure c. The aggregation extent can be estimated by taking the absorption ratio (ALSPR/ATSPR) of LSPR and TSPR peaks of Au nanoparticles, respectively. The absorption ratio (ALSPR/ATSPR) of LSPR and TSPR peaks at A727 and A542 were selected to estimate the extent of agglomeration, respectively. The absorption ratio A727/A542 of the colorimetric assay was plotted in the range from 0.1 to 200 μM, and the calibration curve was fitted to the logistic plot with a regression coefficient of 0.9870 (Y = 0.7033–0.3721 (1 + exp(X – 0.117)/0.429), where Y is the absorption ratio, and X is the concentration of Hg2+. Additionally, a linear relationship among the absorbance ratio and Hg2+ concentration commencing from 0 to 0.10 μM with R2 = 0.9842 (Y = 1.0866X + 0.414), is summarized in the inset of Figure . The limit of detection (LOD) for Hg2+ based on 3σ/S for Au nanoparticles prepared with (14-2-14) GS at a signal-to-noise ratio of 3 was estimated to be 1.8 nM, where σ stands for standard deviation, and S is the slope. The LOD was much lower than the Hg2+ detection limit set by WHO (30 nM) and the U.S. environmental protection agency (10 nM).[3,4] In addition, the obtained LOD is about ∼42 times higher than the Au nanoparticles prepared with GS of a larger alkyl chain length.[22] The limit of detection reported in this work for Au nanoparticles synthesized with (14-2-14) is 1.8 nM higher than the 107, 76, and 176 nM for (16-2-16), (18-2-18), and (20-2-20) GS, respectively.[22] This clearly indicates that the selection of the chain length is critical for obtaining superior sensing properties. However, detail investigation is required to investigate the effect of the chain or spacer length on the detection capability of metal nanoparticles.

Figure 6

(a) Demonstrating UV–vis absorption spectra of Au nanoparticles with different concentrations of Hg2+ ranging from 0 to 200 μM. (b) Represent the change in absorbance vs concentration after the addition of Hg2+. (c) Images of Au nanoparticle solutions with different concentrations of Hg2+.

Figure 7

Logistic plot of ALSPR/ATSPR vs concentrations of Hg2+ranging from 0 to 200 μM in 0.01 M sodium acetate at pH = 4 for Au nanoparticles synthesized. The inset is the plot of concentrations of Hg2+ranging from 0 to 0.10 μM.

The improved detection ability is due to the GS, which acts as a soft template for the anisotropic growth of Au nanoparticles. The surfactant adsorbs on the Au surface (at different crystallographic faces) leading to the preferential growth of nanoparticles. The cationic GS promotes growth of the {111} end facet of Au nanoparticles by preferential adsorption at {100} and {110} side facets.[26] The interaction between Hg2+ and Au nanoparticles only occurs when mercury exist in the Hg0 state. Therefore, conversion or reduction of Hg2+ to Hg0 state is essential. Since the as-prepared nanoparticles are used for detection of Hg2+, therefore, the citrate ions available on the surface of Au can act as the reducing agent, reduce Hg2+ to Hg0 state, and allow interaction between Au–Hg, which also known as amalgamation,[27,28] confirmed by the SPR change and transformation of the solution color from pink to tarnish blue followed by aggregation of Au nanoparticles. Figure S4 schematically describes the interaction between Hg2+ and Au nanoparticles. On the basis of obtained results, it is concluded that the size of metal nanoparticles is no longer critical for the preparation of an efficient selective chemoprobe; rather, growth of more number of edges provides a large number of sights for incoming moieties and plays an important role in improving the detection capability of anisotropic metal nanoparticle irrespective of their large sizes. The most unstable region (high-energy facets) is contributive for the formation of more number of edges and corners, which in turn is favorable for the growth of low-energy facets. These low-energy facets provide a large number of active sites on the surface of nanoparticles for interaction with the heavy metal ion.[17,18,26]

Further, the selectivity of the as-prepared assay for Hg2+ has been compared to other different metal ions, including Cu2+, Cd2+, Fe2+, K+, Mn2+, Ni2+, Pb2+, Sn2+, Zn2+, and Zr2+ with a concentration of 1000 μM for each. Deliberately, the concentration of Hg2+ was maintained at 200 μM. The time-dependent absorption ratio of LSPR-to-TSPR peaks (A727/A542) of Au nanoparticles in the absence and presence of different metal ions is shown in Figure a, whereby a sharp change in the absorption ratio for Hg2+ transpired in a short interval of time in comparison to other metal ions. In addition, the red Au nanoparticle solution was changed to tarnish blue in the presence of Hg2+ ions, whereas no significant change in the color was observed with the passage of time in other cases. The Au nanoparticles showed a visible change in the solution color from red to tarnish blue within 180 s for 100 μM Hg2+ ion followed by saturation in the absorption ratio (ALSPR/ATSPR). The image of the corresponding solutions is shown in Figure b. It is of note that the concentration of Hg2+ used for comparative analysis was about three orders lower than the other metal ion concentrations, which reflects the detection potential of Au nanoparticles synthesized using (14-2-14) GS. In addition, the cross-selectivity of the assay prepared for Hg2+ ions in the solutions was further investigated by mixing the Hg2+ ion already consisting of other different metal ions. The solution color was changed from red to tarnish blue (Figure c) in a similar manner as was observed in the case of Hg2+ ions, again confirming the interaction of Hg2+ with Au nanoparticles irrespective of the presence of other metal ions. In order to estimate the detection ability of anisotropic Au nanoparticles in this work toward Hg2+, the limit of detection is compared with Au nanoparticles of different morphologies shown in Table S2. As is clear, the LOD of anisotropic Au nanoparticles in this work has higher sensitivity toward Hg2+ compare to the different morphologies, which clearly reflects their potential.

Figure 8

(a) Plot of A727/A542 vs time for Au nanoparticles with different metal ions. (b) Images of Au nanoparticle solutions with different metal ions and (c) after the addition of Hg2+ ions.

Conclusions

In this work, the anisotropic Au nanoparticles were synthesized using (14-2-14) GS via the one-step thermal reduction method. The Au nanoparticles consist of different morphologies including round, triangular, rectangular, pentagonal, and hexagonal. The Au nanoparticle-based colorimetric probe was efficient to detect Hg2+ ions with higher detection ability and selectivity without any surface modification of Au nanoparticles with the Hg2+ specific ligand. The Au nanoparticles with well-defined morphologies displayed a detection limit as low as 1.8 nM, much lower than the required world standards. The results suggest that the anisotropic metal nanoparticles are significant to detect heavy metal ions.

Experimental Section

Materials

Chloroauric acid (HAuCl4) and sodium citrate tribasic dihydrate (C6H5O7Na3·2H2O, Sigma-Aldrich) have been used as precursor materials for the synthesis of Au nanoparticles. The shape-directing agent, dimethylenebis-(tetra-decyldimethylammonium bromide) (14-2-14), was prepared in keeping with the protocol reported previously.[25] The high quality of GS was accomplished after a repetitive crystallization process by using a mixture of ethyl acetate and acetone. Sodium acetate (C2H3NaO2), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride (NaCl), and all metal salts (HgCl2, CuCl2, CdCl2, FeCl2, KCl, MnCl2, NiCl2, Pb(C2H3O2), SnCl2, ZnCl2, and ZrCl3 (Sigma-Aldrich)) were used in sensing measurements. The glasswares were cleaned with freshly prepared HCl/HNO3 (3:1, aqua regia), consequently cleaned thoroughly with demineralized water and dehydrated in an oven prior to usage.

Synthesis of Au Nanoparticles

The Au nanoparticles of different morphologies were synthesized by reducing a gold precursor with sodium citrate tribasic dihydrate in the presence of the shape-directing agent (14-2-14). Typically, 50 mL of aqueous solutions of HAuCl4·3H2O (0.25 mM) was boiled at 110 °C. A freshly prepared aqueous sodium citrate (5.0 mL, 2.5 mM) containing (14-2-14) GS solution with a concentration of 0.20–3 mM was added to the gold salt solution and kept under vigorous stirring at 110 °C. Subsequently, the color of the solution gradually changes from pale yellow to red within 30 s. The reaction was allowed to proceed further for 30 min in order to harvest a stable solution. The solution was then left untouched at room temperature for 2 h.

Characterizations

The formation of Au nanoparticles and performing colorimetric sensing measurements have been characterized using UV–visible spectroscopy (Systronics 2202 spectrophotometer) in the spectral range of 450–900 nm. The morphological characterization of Au nanoparticles for size and shape distribution was executed using transmission electron microscopy (TEM, Philips CM-200). The histograms of the particle size and shape distribution were collected from more than 100 particles. Energy-dispersive X-ray spectroscopy (EDXS), attached to the TEM equipment, have been used to examine the chemical composition of Au nanoparticles. The Fourier transformed infrared spectroscopy (FTIR) of synthesized Au nanoparticle solutions was recorded by using an Agilent Cary 630 100 V FTIR instrument, operated in the range of 700–3000 cm–1. The FTIR spectral studies of Au nanoparticles were carried out after washing four times with water using the centrifuge process.

Chemosensor Probe for Detection of Hg2+ Ions

C2H3NaO2 buffer solution (0.01 M) with a pH range from 1.5 to 11 by the addition of 0.1 M HCl and NaOH was prepared to check the stabilization condition of aqueous Au nanoparticles. In addition, to accomplish detection of very low concentrations of metal ions, the NaCl concentration was optimized in aqueous Au nanoparticles, which reduces the electrostatic repulsion between Au nanoparticles and provides a low barrier for metal ion-induced nanoparticle aggregation. The synthesized Au nanoparticles were stable in 0.75 M NaCl concentrations at pH = 4. The optimization process is reported elsewhere.[22] For detection of Hg2+, 1500 μL of aliquots of as-prepared Au nanoparticles were pretreated with 500 μL of NaCl. The prepared solution was then added to aqueous Hg2+ (in the range from 0.1 to 200 μM), which was prepared in 0.01 M sodium acetate buffer solution at pH 4.0. The resulting solution was left for several minutes at room temperature in order to determine preliminary sensing measurement through the naked eye before UV–vis analysis.