Selective and sensitive SERS sensor for detection of Hg²⁺ in environmental water base on rhodamine-bonded and amino group functionalized SiO₂-coated Au–Ag core–shell nanorods

Abstract

Here, we designed a new type of SERS sensor with high selectivity and sensitivity for the determination of Hg²⁺ at picomolar concentrations based on hydrolysis reactions. The Rhodamine 6G-derived Schiff base bonded and amino group functionalized SiO₂-coated Au–Ag core–shell nanorods (Au@Ag@SiO₂–NH₂-R6G, NR-Rs), which show extraordinary enhancement factors (EF) and good stability, were employed as the SERS substrate of the sensor. The tunable ability of the chemical bonding of the substrate generates appropriate probe molecules bound to the surface of the nanorods (NRs) and improves the selectivity and sensitivity of the SERS sensor dramatically. The limit of detection (LOD) of Hg²⁺ obtained using the present SERS sensor was 0.33 pmol L⁻¹. Furthermore, the proposed substrate has pH-responsive ability. The present sensor is suitable for the on-site determination of pH and Hg²⁺ by combining with a portable Raman spectrometer.

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1. Introduction

The technique of surface-enhanced Raman spectroscopy (SERS) is currently undergoing a very dynamic development and many novel directions are rapidly emerging.^1,2 In recent years, SERS has opened up new and exciting horizons in analytical chemistry and molecular biology, such as sensors for detecting organic pollutants and single-molecule detection of proteins.^3–5 Now it is widely accepted that there are two main effects of enhancement mechanisms of SERS: the electromagnetic effect (EM) and the chemical effect (CM). The localized surface plasmon resonance (LSPR) plays a key and dominant role for the EM in the overall enhancement; whereas the CM, which is due to the interaction of the molecules adsorbed on the metal with the metal surfaces, is mostly from the first layer of the charge-transfer resonance between the adsorbate and the metal.^6–8

Over the past 30 years, a large variety of highly SERS-active substrates have been developed, such as metal nanoparticles,^9–11 roughened electrodes,¹² metal island films^13–15 and so on. Metal nanoparticles, particularly Au and Ag nanoparticles are frequently used as SERS substrates because of the simplicity of their preparations and characterizations.^16–18 Previous research found that bimetallic Au and Ag nanoparticles could form favorable states of aggregation and increase the SERS activity more significantly than mono- Au or Ag nanoparticles.^19–21 Metal nanoparticles with various shapes, including cubes, prisms, rods and octahedra, have been synthesized as SERS substrates.²² Experimental and theoretical results indicate that anisotropic nanoparticles have stronger enhancement effects than spherical nanoparticles because the sharp edges and angles of metal nanoparticles will give extra enhancements owing to the “lightning rod effect”.²³ In order to improve the selectivity and stability of SERS-active substrates, Tian et al.²⁴ developed a new technique called shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), by employing a kind of metal nanoparticles with insulating ultrathin thin SiO₂ shells possessing pinholes on their surface as SERS substrates. The thin coating keeps nanoparticles from agglomerating, and protects the SERS-active nanostructure from contacting with whatever is being analyzed. Compared with routine metal nanoparticles, metal@SiO₂ nanoparticles have higher sensitivity, better stability and reproducibility.^25,26

Mercury ions (Hg²⁺) can lead to damages of brain, kidneys, the central nervous system, the immune system and the endocrine system of human body even at a very low concentration.²⁷ Soluble Hg²⁺ in drinking waters are the main source of mercury absorbed into human body and attract great concern.²⁸ U. S. Environmental Protection Agency (EPA) recommends that the levels of Hg²⁺ in drinking water should be reduced to 2 μg L⁻¹ (10 nmol L⁻¹).²⁹ Thus, there is an urgent need of accurate and ultrasensitive detection methods for Hg²⁺.

Nowadays, several Hg²⁺ detection methods based on inductively coupled plasma-mass spectrometry (ICP-MS),³⁰ fluorescence quenching,³¹ surface plasmon resonance (SPR) sensor³² and SERS³³ have been developed. SERS, in particular, is a promising method for ultra-highly sensitive detection of Hg²⁺ because it offers practical facilities for on-site detection by using miniaturized Raman instruments.³⁴ Unlike organic or biological molecules, Hg²⁺ does not exhibit any Raman signals, thus the only way to determine it using SERS probe is the indirect signal readout. Recently, several SERS probe for the determination of Hg²⁺, such as cysteine,³⁵ tryptophan,³⁶ 4,4′-dipyridyl,³⁷ and DNA³⁸ were proposed by many research groups. In these researches, nanoparticles and probes were usually self-assembled by physical absorption.

In this paper, we try to use Rhodamine 6G (R6G)-derived Schiff base bonded and amino group functionalized SiO₂-coated Au–Ag core–shell nanorods (NR-Rs) as the SERS substrate. It has been reported that R6G and its derivatives can be used as fluorescence³⁹ and SERS^40,41 probes. However, SERS sensors are more selective than fluorescence sensors in general due to the abundant information of molecular structures in Raman spectra. R6G possesses functional groups of chemical affinity, and exhibits strong Raman signals as SERS probe.⁴² The tunable ability of chemical bonding of the substrate will make appropriate probe molecules bound to the surface of nanorods and improve selectivity and sensitivity of the SERS sensor dramatically.⁴³ The proposed substrate was applied to the determination of pH and Hg²⁺ by combining with a portable Raman spectrometer.

2. Experimental details

2.1 Chemicals and reagents

Rhodamine 6G (R6G, 98.5%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O, 49.0% Au) and silver nitrate (AgNO₃, 99%) were purchased from J&K Chemical Ltd. 3-Aminopropyltriethoxysilane (APTES, 99%) and sodium silicate solution (Na₂SiO₃, 27% SiO₂) were purchased from Sigma-Aldrich Ltd. Hydrazine monohydrate (N₂H₄·H₂O, 85%), oxalaldehyde hydrate (40%), sodium borohydride (NaBH₄, 96%), hexadecyl trimethyl ammonium bromide (CTAB, 98%), ascorbic acid (AA, 99%), sulfuric acid (H₂SO₄, 98%), nitric acid (HNO₃, 65%), hydrochloric acid (HCl, 38%), phosphorous acid (H₃PO₄, 85%), sodium hydroxide (NaOH, 98%), ammonia solution (25%), Hg(NO₃)₂, CuSO₄·5H₂O, Cr(NO₃)₃·9H₂O, Pb(CH₃COO)₂·3H₂O, FeCl₃·6H₂O, MgCl₂·6H₂O, NiCl₂·6H₂O, CoCl₂·6H₂O, ZnCl₂, CdSO₄·8/3H₂O and BaCl₂·2H₂O were purchased from Sinopharm Chemical Reagent Co., Ltd., China. All of these reagents were used without further purification. All the solvents were of analytical grade. All aqueous solutions were prepared with ultra-pure water (18.2 MΩ cm) from a Milli-Q water purification system.

2.2 Instruments

Raman spectra were obtained using a portable miniature Raman spectrometer (Changchun Jilin University Little Swan Instruments Co., Ltd., Changchun, China) with 1 cm quartz cells and a 785 nm diode laser. The laser power was chosen as 300 mW. The surface morphologies of the NRs were measured on a Hitachi H-800 transmission electron microscope (TEM, Hitachi Ltd., Japan) operating at 175 kV and a JEM-2100F high resolution transmission electron microscope (HR-TEM, JEOL Ltd., Japan) operating at 200 kV. The samples for TEM images were prepared by dropping the solutions containing NRs on the carbon-coated copper grid and drying at room temperature. Absorption spectra of the NRs colloids were recorded on a TU-1810C UV-Vis spectrometer (Beijing Purkinje General Inc., China) within the wavelength range from 300 to 1000 nm. Spectra of ¹H-NMR were measured on a Mercury 300BB nuclear magnetic resonance spectrometer (Varian Inc., USA) with chemical shifts reported as ppm (in d⁶-DMSO, TMS as internal standard). ESI-MS spectra were measured on a LC/MS QTRAP spectrometer (AB SCIEX Inc., USA). High resolution ESI-TOF-MS (HR-ESI-TOF-MS) spectra were measured on a HPLC-ESI-TOF-MS spectrometer (AB SCIEX Inc., USA). FT-IR spectra were recorded using KBr pellets on a Nicolet Avatar 360 FT-IR spectrophotometer (Thermo Fisher Scientific Inc., USA) in the wavenumber region of 4000–400 cm⁻¹. Fluorescent spectra were collected with a RF-5301 PC fluorospectrophotometer (SHIMADZU CO., LTD., Japan). All pH measurements were made with a PHS-3C pH-Meter (INESA Scientific Inc., China).

2.3 Synthesis of SERS substrate NR-Rs

The synthetic route of NR-Rs is shown in Fig. 1.

Fig. 1 (a) Schematic illustration of the formation process of Au@Ag@SiO₂–NH₂ NRs. (b) Synthetic route of compound R–CHO. (c) Synthetic route of SERS substrate NR-Rs.

2.3.1 Synthesis of Au@Ag@SiO₂–NH₂ NRs. Au NRs were synthesized by using a seed mediated growth procedure.⁴⁴ CTAB-capped Au seeds were synthesized by the chemical reduction of HAuCl₄ with NaBH₄. 7.5 mL of 0.1 mol L⁻¹ CTAB was mixed with 100 μL of 24 mmol L⁻¹ HAuCl₄. The mixture was diluted with ultrapure water to 9.4 mL, and was stirred with a magnetic stirrer in ice-bath. Then, 600 μL of 10 mmol L⁻¹ NaBH₄ was added. The Au seeds were obtained. The growth solution for the formation of the Au NRs consisted of 100 mL of 0.1 mol L⁻¹ CTAB, 2.04 mL of 24 mmol L⁻¹ HAuCl₄, 2.0 mL of 0.5 mol L⁻¹ H₂SO₄, 1.0 mL of 10 mmol L⁻¹ AgNO₃ and 800 μL of 0.1 mol L⁻¹ AA. 240 μL of Au seed solution was added to the growth solution to initiate the growth of the Au NRs. After 12 h, the growth was stopped. The obtained Au NRs were purified by centrifuging the result solution at 13 000 rpm for 10 min, twice. The nanorods were collected and redispersed in ultrapure water with the volume of the original solution before centrifuging.

20 mL of the above Au NRs solution and 3.0 mL of 0.1 mol L⁻¹ AA were mixed in a 100 mL round flask under magnetic stirring. Then 7.0 mL of 1 mmol L⁻¹ AgNO₃ was added into the mixture in drop wise manner at a rate of 50 μL per 30 s. AgNO₃ was reduced by AA and the resultant Ag continuously grew on the surface of Au NRs. After its color changed from purplish red to brown, the solution was continuously stirred for 30 min. The obtained Au@Ag NRs were purified by centrifuging the result solution at 13 000 rpm for 10 min, twice. The result nanorods were also collected and redispersed in ultrapure water with the volume of the original solution before centrifuging.

Amino group functionalized SiO₂-coated Au@Ag NRs (Au@Ag@SiO₂–NH₂ NRs) were prepared according to the reported method, with some modification.⁴⁵ A freshly prepared aqueous solution of 0.4 mL of 1 mmol L⁻¹ APTES was added to 30 mL Au@Ag NRs solution under vigorous stirring in 15 min for the complexation of amine groups with the silver surfaces of Au@Ag NRs. Then 3.2 mL of 0.54% (w/w) Na₂SiO₃ was added to the solution under vigorous stirring. The mixture was stirred for 20 min in a water bath at 90 °C. The obtained Au@Ag@SiO₂ NRs were purified by centrifuging the result solution at 13 000 rpm for 10 min and dispersed in 30 mL ethanol containing trace amounts of water. Then 1 mL of 0.1 mmol L⁻¹ APTES was added to the dispersion and the mixture was heated to 50 °C for 8 h to modify the silica surfaces of NRs. The Au@Ag@SiO₂–NH₂ NRs were isolated from the mixture by centrifuging at 13 000 rpm for 10 min and washed with ethanol and water for 3 times. Finally, the Au@Ag@SiO₂–NH₂ NRs were dispersed in 30 mL ethanol and stored at 4 °C.

2.3.2 Synthesis of compound R–CHO. R–NH₂ was synthesized from R6G according to the literature method.⁴⁶ 0.958 g R6G was dissolved in 30 mL absolute ethanol. 3.0 mL (excess) N₂H₄·H₂O was added, and the mixture was heated to reflux in an oil bath for 2 h with stirring. The precipitate produced was filtered and washed with 15 mL EtOH/H₂O (1 : 1) for 3 times, and dried over P₂O₅ under vacuum. The reaction yielded 0.77 g R–NH₂ as light pink solid (yield 90%). ESI-MS: m/z 429.4 for [M + H]⁺. ¹H-NMR (d⁶-DMSO, 300 MHz), δ (ppm): 7.80–7.71 (m, 1H), 7.51–7.41 (m, 2H), 6.98–6.88 (m, 1H), 6.27 (s, 2H), 6.10 (d, J = 0.7 Hz, 2H), 5.00 (t, J = 5.4 Hz, 2H), 4.22 (s, 2H), 3.24–3.03 (m, 4H), 1.87 (s, 6H), 1.21 (t, J = 7.1 Hz, 6H).

R–CHO was synthesized according to the reported method, with some modification.⁴⁷ 0.30 g R–NH₂ was dissolved in 15 mL of boiled absolute ethanol. Then 3.0 mL (excess) oxalaldehyde hydrate was added, and the mixture was heated to reflux in an oil bath under N₂ atmosphere for 4 h with stirring. The precipitate produced was filtered and washed with 15 mL cold ethanol for 3 times, and dried over P₂O₅ under vacuum. The reaction yielded 0.28 g R–CHO as yellow solid (yield 85%). ESI-MS: m/z 469.3 for [M + H]⁺. ¹H-NMR (d⁶-DMSO, 300 MHz), δ (ppm): 9.25 (d, J = 7.5 Hz, 1H), 8.00 (d, J = 7.0 Hz, 1H), 7.63 (dtd, J = 22.0, 7.4, 1.1 Hz, 2H), 7.26 (d, J = 7.5 Hz, 1H), 7.04 (d, J = 7.4 Hz, 1H), 6.31 (s, 2H), 6.18 (d, J = 0.5 Hz, 2H), 5.18 (t, J = 5.4 Hz, 2H), 3.14 (dd, J = 6.9, 5.6 Hz, 4H), 1.85 (s, 6H), 1.20 (t, J = 7.1 Hz, 6H).

2.3.3 Synthesis of NR-Rs. NR-Rs were synthesized by a direct chemical bonding reaction, which is also a Schiff base condensation reaction between R–CHO and Au@Ag@SiO₂–NH₂ NRs. 20 mL of Au@Ag@SiO₂–NH₂ NRs ethanol solution was mixed with 20 mL of 1.0 × 10⁻⁵ mol L⁻¹ R–CHO ethanol solution, and the mixture was stirred thoroughly and protected from light for 24 h at room temperature. The obtained NR-Rs were purified to remove unreacted R–CHO by centrifuging at 13 000 rpm for 10 min and washed with copious ethanol and water sequentially for 3 times. Finally, the NR-Rs were dispersed in 20 mL ultrapure water and stored at 4 °C.

2.4 SERS measurements

In a typical process, 100 μL of NR-Rs solution, 10 μL of analyte solution and 50 μL of phosphate buffer were added into a quartz cell for SERS analysis. The typical exposure time for each measurement was 10 s with 3 accumulations. Five measurements of three samples were used to calculate the relative standard deviations (RSDs). Three environmental water samples, including lake water, reservoir water and river water, were collected from local sources of drinking water. These samples and their spiked controls were directly analyzed by SERS without any preparation procedures.

3. Results and discussion

3.1 Characterization of Au@Ag@SiO₂–NH₂ NRs

Fig. 1a shows the preparation process of Au@Ag@SiO₂–NH₂ NRs. The Au NRs prepared by using a seed mediated growth procedure⁴⁴ exhibit two main absorption bands: one is the transverse surface plasmon resonance (SPR) band around 520 nm, similar to the absorption band of spherical Au nanoparticles; the other is the longitudinal localized surface plasmon resonance (longitudinal LSPR) band, which is tunable from the visible through near-infrared region with the changing of nanorod aspect ratio.⁴⁸ The as-prepared Au NRs with typical lengths of 45 nm and aspect ratios about 3.0 show a strong longitudinal LSPR band around 674 nm, and a weak transverse SPR band around 516 nm (Fig. 2a and e). The Au@Ag NRs were prepared by chemical reduction method. AgNO₃ was reduced by ascorbic acid (AA) and the resultant Ag continuously grew on the surfaces of Au NRs. The average aspect ratios, length and thickness of silver shells of the Au@Ag NRs are 2.5, 55 nm and 5 nm, respectively (Fig. 2b and e). Compared with Au NRs, Au@Ag NRs produce a new LSPR band of Ag shells at 390 nm, and a LSPR band shifted to 596 nm. In order to obtain selective and stable SERS substrates, amino group functionalized silica shells with a thickness of 4 nm were coated on the surface of Au@Ag NRs (Fig. 2c and d). The silica shells, which have little effects on the assignments of LSPR bands of Au@Ag NRs (Fig. 2e), can reduce the agglomeration of nanoparticles. The FT-IR absorption spectrum of the Au@Ag@SiO₂–NH₂ NRs is shown in Fig. S1 (see ESI†). The characteristic absorption bands in the range from 3500 cm⁻¹ to 3200 cm⁻¹ were attributed to asymmetrical stretching vibrations of amino groups. The band at 1056 cm⁻¹ was attributed to stretching vibrations of Si–O–Si structures.⁴⁹ These absorption bands indicated that silica shells functionalized with amino groups were successfully coated on the surface of Au@Ag NRs. The Au@Ag@SiO₂–NH₂ NRs could remain stable for at least 20 day at room temperature. In contrast, obvious agglomeration and adherent phenomenon were observed for Au NRs and Au@Ag NRs in 5 days.

Fig. 2 TEM images of Au NRs (a), Au@Ag NRs (b), and Au@Ag@SiO₂–NH₂ NRs (c); HR-TEM image of Au@Ag@SiO₂–NH₂ NRs (d); UV-Vis absorption spectra of Au NRs, Au@Ag NRs, Au@Ag@SiO₂–NH₂ NRs (e).

3.2 Calculation of SERS enhancement factor

The SERS enhancement factor (EF) is an important parameter for the performance demonstration of SERS substrates. The strong Raman peaks at 1512 cm⁻¹ of 1.0 mmol L⁻¹ and 1.0 nmol L⁻¹ R6G solution were chosen as the representation peaks for EF calculation. Herein, the SERS EF is calculated from the standard equation defined as:where I_SERS and I_Raman correspond to the intensities of diagnostic bands in SERS and unenhanced Raman spectra of R6G, respectively; C_SERS and C_Raman are, respectively, the concentrations of R6G solutions for SERS and unenhanced Raman analyses. Other experimental conditions, such as the exposure time and the power of laser, are identical in all cases. The results are shown in Fig. 3. Using Au@Ag@SiO₂–NH₂ NRs as SERS substrate, the Raman peak area at 1512 cm⁻¹ of 1 nmol L⁻¹ R6G obtained increase 2.0 and 12.9 times of the peak areas obtained using SERS substrate of Au NRs and Au@Ag NRs, respectively. The SERS activity of Au@Ag@SiO₂–NH₂ NRs was improved because the pinhole of silica shells make SERS-active nanostructure contact sensitively with target analytes.⁵⁰

Fig. 3 (A) SERS spectra of 1 nmol L⁻¹ R6G using Au@Ag@SiO₂–NH₂ NRs (a), Au@Ag NRs (b), Au NRs (c) as SERS substrates, and unenchanced Raman spectrum of 1 mmol L⁻¹ R6G (d); (B) Raman peak areas and EFs at 1512 cm⁻¹.

3.3 Effect of H⁺ on SERS intensity and calculation of acidity constants

Fig. 4 shows the responses of the present SERS sensor at different pH values in phosphate buffer containing a few chlorine ions (50 mmol L⁻¹). With the increase of pH values, the intensity of Raman band at 1512 cm⁻¹ was observed to decrease distinctly, especially in the pH range from 2.0 to 7.0. It is well known that R6G-derivative exists isomers with different chemical structures and spectroscopic properties, such as spirocyclic and open-cycle forms.^51,52 In an alkaline environment, most of the –R groups of NR-Rs exist in spirocyclic form, which make less –R groups fall to the “hot spot” of the NRs and the substrates exhibit weak SERS enhancement. On the basis of the surface selection rule, when molecules are perpendicular on the substrate, the SERS signal is stronger.⁵³ Therefore, more H⁺ will make more spirocycles open, and will enable protonated –R groups to move closer to the surfaces of NRs, which will enhance the SERS signals. When the concentration of H⁺ makes the protonation tend saturated, the maximum SERS intensity of the present sensor is obtained typically.

Fig. 4 SERS intensity at 1512 cm⁻¹ at different pH values.

The acidity constants K_a of NR-Rs were determined by SERS titration (Fig. 4). Sigmoidal curve fitting of Henderson–Hasselbach-type mass action eqn (2) to the SERS intensity I recorded as a function of pH yielded the values of pK_a,

where I is the observed SERS intensity at a fixed Raman shift (1512 cm⁻¹), I_max and I_min are the corresponding maximum and minimum respectively. A pK_a1 of 3.13 and a pK_a2 of 6.38 were yielded, which indicated that the present SERS sensor is valuable for pH sensing in acidic and near neutral media.

3.4 Effect of metal ions on SERS intensity

Thirteen metal ions including Na⁺, K⁺, Mg²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cr³⁺, Cd²⁺, Fe³⁺, Pb²⁺, Ba²⁺, Zn²⁺ and Hg²⁺ at the concentration of 1 μmol L⁻¹ were employed to evaluate the coordinative properties of the NR-Rs by competition experiments in phosphate buffer (pH = 1.94). As seen in Fig. 5, no significant spectral changes occurred in the presence of all the metal ions except Hg²⁺. The presence of Hg²⁺ resulted in a remarkable decrease of SERS intensity. To explore the mechanism of the decrease, the mixing solution of NR-Rs and Hg²⁺ was centrifuged and the supernatant was analyzed by fluorescent spectrometry and HR-ESI-TOF-MS spectrometry. An obvious fluorescence emission at 552 nm (excitation was at 520 nm) and a peak at m/z 415.2007 corresponding to [R6G acid + H]⁺ were observed (see ESI Fig. S2 and S3†), which indicated that in the presence of Hg²⁺, the complex of NR-R hydrolyzed to the R6G acid, a strong fluorescent substance that could be dissolved in the supernatant (Fig. 6). Certainly, the SERS activity of the NR-Rs would be decreased due to the detachments of –R groups occurred by Hg²⁺, which could facilitate the determination of Hg²⁺.

Fig. 5 SRES intensity with different metal ions at 1512 cm⁻¹ (pH = 1.94).

Fig. 6 Schematic illustration for determining Hg²⁺ using NR-Rs.

3.5 Determination of Hg²⁺

100 μL of NR-Rs, 10 μL of Hg²⁺ solution and 50 μL of phosphate buffer (pH = 1.94) were selected as the experimental conditions for quantitative assay of Hg²⁺. Linear relationship between concentrations of Hg²⁺ and SERS intensities was attained in the concentration range of Hg²⁺ from 1 pmol⁻¹ to 0.1 μmol L⁻¹. The regression equation of calibration curve is I/I₀ = −0.051 lg C + 0.67 (C: μmol L⁻¹), where I and I₀ are the SERS peak areas at 1512 cm⁻¹ of NR-Rs in the presence and absence of Hg²⁺, respectively. The correlation coefficient and the LOD were 0.9739 and 0.33 pmol L⁻¹, respectively. These results showed that NR-Rs could be used as an ultrasensitive and selective SERS substrate for the Hg²⁺ sensor. The present sensor was applied to the determination of Hg²⁺ in environmental water samples. The obtained results were consistent with those obtained using atomic absorption spectrometry (AAS) (Table 1). Compared with AAS, the present SERS sensor for Hg²⁺ detection in aqueous samples has the advantages of easy and simple manipulation, as well as the ability of rapid on-site analysis with portable Raman spectrometers.

Table 1 Detection and recovery test of Hg²⁺ in water samples (n = 3)

Sample	AAS method (pmol L^−1)	SERS method (pmol L^−1)	Spiked Hg^2+ (pmol L^−1)	Measured Hg^2+ (pmol L^−1)	Recovery (%)
Lake water	59.80	60.17	10.00	71.35	101.68
Reservoir water	47.38	50.05	10.00	62.45	104.00
River water	65.15	68.34	10.00	79.58	101.58

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4. Conclusions

In summary, we prepared a new type of SERS sensor based on the substrates of Rhodamine-bonded and amino group functionalized SiO₂-coated Au–Ag core–shell nanorods (NR-Rs), which exhibit ultra-sensitivity for Hg²⁺ detection and pH-responsive ability. The characteristic hydrolysis process of the complex of NR-R was identified and employed for the determination of Hg²⁺ in environmental water at picomolar concentrations. The consistent results obtained by the present SERS sensor and AAS, and the stability of the SERS substrates of the present sensor, show the potentials of the practical application of the present sensor in the rapid and on-site analysis.

Acknowledgements

This work was supported by National “Twelfth Five-Year” Plan for Science and Technology Support (no. 2012BAF14B08), National Natural Science Foundation of China (nos 21405057, 21207047, 21075049 and 21105037), Science and Technology Developing Foundation of Jilin Province (nos 20121808 and 20125006), and Scientific Forefront and Interdisciplinary Innovation Project of Jilin University (no. 450060481018).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04423e

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