Introduction

Some metal ions are crucial components of living organisms, serving as cofactors for numerous proteins with diverse functions (Yang et al. 2016). Nickel is an essential trace element to biota for its biological functions. It can form complex with amino acids, peptides, phosphates, and other biotic ligand models like nucleic acids (Ragsdale 2008), and it is a biologically important metal ion (Tedsana et al. 2015). Nickel not only stimulates certain enzymes but also plays important roles in various enzyme activities such as hydrogenases, superoxide dismutases, and catalytic processes. However, the amount of nickel we need is very little; an excess of nickel ion in either organism may be thus a toxic metal from biological point of view resulting in pneumonitis, asthma, lung cancer, and also certain disorder of the central nervous system as well as the respiratory system (Deblina et al. 2016). Hence, Ni2+ detection and quantification is of considerable significance (Li et al. 2009).

There are various analytical methods for detection of Ni2+ such as atomic absorption spectrometry (Sun et al. 2006), microwave-induced plasma atomic emission spectrometry (Jankowski et al. 2005), membrane- and potentiometric-based techniques (Gupta et al. 2007), fluorescence (Ganjali et al. 2015), electrochemistry, and chromatography (Tedsana et al. 2015). These methods are sensitive and reliable, but involve high cost, sophisticated instrumentations, and are time-consuming. Therefore, it is important to develop simple, sensitive, and selective methods for determination of trace amounts of nickel.

Nowadays, colorimetric methods, in particular, are extremely attractive because the detection results can be easily read out with the bare eyes. Moreover, they offer advantages of simplicity, rapidity, cost effectiveness, and no requirement of any sophisticated instrumentation. Gold and silver nanoparticles (AuNPs/AgNPs) have been widely adopted as colorimetric sensors to provide alternative schemes to conventional detection methods for metal ions (Kumar and Anthony 2014). AuNPs/AgNPs of various sizes and shapes can be synthesized and surface-modified with different functionalities. Furthermore, they exhibit excellent biocompatibility and unique optoelectronic and chemical properties (Pourreza et al. 2015; Yin et al. 2002; Chen et al. 2014a, b; Noh et al. 2015; Chen et al. 2014a, b). Many works related to the detection of Ni2+ by Au/Ag nanoparticle-based colorimetric assays have been reported. Li ҆s group demonstrated the glutathione-modified AgNPs to detect Ni2+ (Li et al. 2009). Zhang ҆s group also developed the peptide-modified AuNPs for parallel detection of Cd2+, Ni2+, and Co2+ (Zhang et al. 2012). Annadhasaan ҆s group reported a method for Ni2+ and Co2+ based on green synthesis of AuNPs under sunlight irradiation (Annadhasan et al. 2015). The detailed comparison is organized in Table S1. As summarized, these methods’ sensitivity and selectivity need to be improved. Therefore, establishing methods to detect Ni2+ sensitively and selectively is still demanding.

Compared to gold, silver is relatively abundant and inexpensive, and AgNPs are popular for the outstanding chemical and physical properties, such as size- and shape-depending optical, electrical, biological, and magnetic properties (Isabel et al. 2011; Pourreza et al. 2015). AgNPs are also widely used in various fields such as catalysis (Joseph and Mathew, 2015), optics (Kang et al. 2014), imaging (Tai et al. 2007), cancer therapy (Austin et al. 2014; Carolina et al. 2014), and sensor technology (Filippo et al. 2013; Rameshkumar et al. 2014). Obviously, the distance-dependent surface plasmon resonance (SPR) absorption of AgNPs have become a useful tool for the development of colorimetric sensing with various analytes. We develop a simple colorimetric nanosensor for the determination of Ni2+ by using the interparticle plasmon coupling on analyte-induced aggregation of AgNPs modified with two specific recognition ligands, sodium dodecyl sulfonate (SDS), and adenosine monophosphate (AMP). SDS and AMP were both commercially available, and they were bound onto the AgNPs through the sulfonic group and amino group. Owing to cooperative effect of electrostatic interaction between Ni2+ and SDS and coordination interaction between Ni2+ and AMP, well-dispersed AMP-SDS-AgNPs were readily aggregated in the presence of Ni2+. And consequently, the color of AMP-SDS-AgNPs underwent dramatic change from bright yellow to orange. According to the color change, the concentrations of Ni2+ can be easily estimated by bare eyes without the aid of the sophisticated and expensive instruments. Scheme 1 outlines the possible sensing mechanism.

Scheme 1
scheme 1

Colorimetric assay of Ni2+ based on the aggregation mechanism using AMP-SDS-AgNPs

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Experimental

Materials and instruments

All chemicals used were of analytical grade and were used without further purification. Silver nitrate (AgNO3), sodium borohydride (NaBH4), adenosine monophosphate (AMP), sodium dodecyl sulfate (SDS), NaOH, HCl, and compounds of the metallic ions (AlCl3, BaCl2, CaCl2, CdCl2, CoCl2, CrCl3, Cu(NO3)2, FeCl3, HgCl2, KCl, MgCl2, MnCl2, NaCl, NH4Cl, ZnSO4, Pb(NO3)2, FeSO4, and NiCl2) were purchased from Shanghai Qingxi Technology Co., Ltd. (Shanghai, China, www.ce-r.cn/sites/qingxi/). Milli-Q purified ultrapure water was used to prepare all the solution tested. All the glassware used must thoroughly be washed with freshly prepared aqua regia (3:1; HCl/HNO3) and then rinsed comprehensively with ultrapure water and air-dried prior to use.

Ultraviolet-visible (UV-vis) absorption spectra were examined on a UV-2550 spectrophotometer (Shimadzu, Kyoto, Japan) using a 1.0-cm quartz cell at room temperature. Transmission electron microscope (TEM) was recorded by a JEM-2100 transmission electron microscope (JEOL Ltd. Japan, www.jeol.cn). Fourier transform infrared (FT-IR) spectra were recorded with KBr pellets on a Nicolet 5700 FT-IR Spectrometer (Nicolet, USA, www.thermonicolet.com). The data of dynamic light scattering (DLS) were obtained on NPA152 nanoparticle size analyzer (Microtrac Inc., USA, www.malvern.com/en/).

Synthesis of AMP-SDS-AgNPs

The AgNPs were prepared according to the previously reported method (Li et al. 2010) in which the AgNO3 was reduced to Ag using NaBH4. Briefly, in a flask, 2.0 mL of 0.01 M AgNO3 was added into 96.8 mL doubly distilled water. Then 8.8 mg of NaBH4 was quickly added into the above mixture solution under vigorous stirring for 30 min at room temperature (25 ± 2 °C); 1.0 mL of 0.01 M AMP and 0.2 mL of 0.01 M SDS were then added into the above aqueous solution. The resulting yellow silver colloidal solution was stirred for 2 h in the dark room. The resulting yellow silver colloidal solution was stored at 4 °C in the dark before use.

Colorimetric detection of Ni2+

In a typical experiment, 2.0 mL of AMP-SDS-AgNPs was mixed with different Ni2+ concentrations in the range of 0 to 60 μM. Finally, the color changes were detected by the bare eye and by UV-vis absorption spectra. The photographs were taken with a digital camera, and the UV-vis spectra were recorded between 300 and 800 nm. The absorbance ratio at 515 and 396 nm (A515nm/A396nm) has been used as the index parameter of AMP-SDS-AgNPs for detection of Ni2+ and the numerator and the denominator represent the degree of dispersion and aggregation, respectively. All the experiments were performed in triplicate. Limit of detection (LOD) values was calculated using the following equation: LOD = 3 S 0/K, where S 0 is the standard deviation of blank measurements (n = 10) and K is the slope of calibration line.

Results and discussion

Response of AMP-SDS-AgNPs to Ni2+

Figure 1a shows absorption spectral change of AMP-SDS-AgNPs upon addition of Ni2+; the absorbance at 396 nm of AgNPs which was ascribed to plasmon band decreased and a new absorption band at 515 nm appeared. Meanwhile, the color changed from yellow to orange (inset of Fig. 1a). The above changes were attributed to the aggregation of AMP-SDS-AgNPs, which was also supported by TEM graph changes. Figure 1b shows the TEM image of well-dispersed NPs, and Fig. 1c presented the TEM image of NPs after the addition of Ni2+ where the particles were found agglomerated and the average size of the NPs was much larger than dispersed NPs. In addition, the DLS measurements were used to monitor the change in the average hydrodynamic size of AMP-SDS-AgNPs. As shown in Fig. S1, after the addition of Ni2+, the size of NPs was greatly increased from 9.8 ± 3.5 to 58.3 ± 15.3 nm. The result obtained with DLS was similar to TEM measurement, also suggesting that Ni2+ can result in the effective aggregation of AMP-SDS-AgNPs.

Fig. 1
figure 1

a UV-vis spectra of AMP-SDS-AgNPs in the absence and presence of 30 μM Ni2+ (inset image is colorimetric response of AMP-SDS-AgNPs to Ni2+), and TEM images of AMP-SDS-AgNPs in the absence (b) and presence (c) of 30 μM Ni2+

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Detection mechanism for Ni2+

To gain insight into the interactions between various functional groups of SDS, AMP, and AgNPs, FT-IR spectra measurements were carried out. Figure S2 compared the characteristic stretching frequencies for SDS, AMP, and AMP-SDS-AgNPs. It is noteworthy that the S=O stretching vibration band at 1260–1109 cm−1 of sulfonic group is obvious for SDS and disappears for AMP-SDS-AgNPs. Furthermore, the band at 900–650 cm−1 assigned to the characteristic peaks of C–N stretching vibrations is clearly shown in the spectrum of AMP and disappears for AMP-SDS-AgNPs. Based on the FT-IR spectra, we concluded that SDS and AMP were co-modified on the surface of AgNPs through the sulfonic group and the amino group.

Ni2+ is known to bind well to groups or ligands containing lone pair electron such as –NH2, imidazole, etc. via the coordination bond (Deblina et al., 2016; Kang et al. 2017). Therefore, it may be the N of imidazole in the AMP on the outer surface of the AMP-SDS-AgNPs that was responsible for sensing Ni2+ based on metal–ligand interactions to form a stable complex. It is noted that electrostatic interaction between Ni2+ and SDS also play an important role, as demonstrated below. Thus, a fast and efficient interparticle cross-linking of the AgNPs occurred in the presence of Ni2+.

Optimization of detection conditions

Effect of AMP concentration

Parameter ascertainment and optimization of the present method are the key factors to its performance in terms of effectiveness and sensitivity, which strongly depend on the AMP concentrations. Since SDS only acts as stabilizer, we mainly investigate the effect of the ratio of the AgNPs and AMP. Therefore, AMP at different concentrations was added to the AgNPs. As shown in Fig. S3, as the ratio of AgNPs and AMP rises, the degree of AgNPs aggregation increases, as is reflected by the ratio of absorbance values at 515 and 396 nm (A515nm/A396nm), while an obvious change in A515nm/A396nm cannot be observed when the ratio of the AgNPs and AMP was higher than 2:1. In contrast, when the molar ratio of AgNPs and AMP was lower than 2:1, both the homogeneity of the AgNPs and the capability of Ni2+ detection became worse. Based on these results, the ratio of the AgNPs and AMP was chosen to be 2:1 in the following experiments.

Effect of pH and reaction time

It is significant to choose optimal reaction conditions that determine enough aggregation of AgNPs, so sample pH value and reaction time were also investigated.

The aggregation of silver nanoparticles can be influenced by factors such as media pH. Therefore, the pH condition for the Ni2+ detection method was optimized in the range of 4.0–11.0. Media pH was adjusted using 1.0 M NaOH and 1.0 M HCl. The molar ratio (A515nm/A396nm) of AMP-SDS-AgNPs showed no obvious change with the pH value increasing from 4.0 to 7.0 after the addition of 30 μM Ni2+. By contrast, the absorbance ratio (A515nm/A396nm) of AMP-SDS-AgNPs changed evidently when the pH value is more than 8.0 (Fig. S4a). The result indicated that AMP-SDS-AgNPs can be more sensitive in the presence of Ni2+ under alkaline environment. Since the pH value of the prepared AMP-SDS-AgNPs was 9.5, to save the time and cost, pH 9.5 which was the condition of the prepared SAA-AgNPs without any pH adjustments was selected.

To determine the interaction time required for aggregation of nanoparticles, change in absorption spectra of silver nanoparticles after addition of Ni2+ was observed and absorbance ratio (A515nm/A396nm) was plotted against time. As shown in Fig. S4b, for the three concentrations that had been examined, it is remarkable that the absorbance ratio of AMP-SDS-AgNPs increased rapidly with time once the Ni2+ was added to the AMP-SDS-AgNPs solution and reached a relatively constant value in 5 min, which was in good agreement with the color change observed by the bare eyes, revealing that the aggregation of AMP-SDS-AgNPs was able to be completed within 5 min under this condition.

Selectivity of AMP-SDS-AgNPs

Specific recognition is an important criterion by which to evaluate the performance of the colorimetric sensor for Ni2+ detection. The effects of common existing metal ions such as Cd2+, Al3+, Ba2+, Ca2+, Cu2+, Ag+, Co2+, Cr3+, Fe3+, Mg2+, Hg2+, K+, Mn2+, NH4 +, Zn2+, Fe2+, Cr2+, and Pb2+ were investigated, in which all the metal ions’ concentration is 30 μM. Figure S5 showed the UV-vis absorption spectra and photographic images of AMP-SDS-AgNPs solution after adding various metal ions. It was found that only the presence of Ni2+ can induce distinct changes of AMP-SDS-AgNPs both in absorption spectrum and the color of the solution change from yellow to orange. These results indicate that the present method can be potentially used to detect Ni2+ in aqueous solution with high selectivity.

To further demonstrate the assay for highly selected colorimetric visualization of Ni2+, we had employed AMP-AgNPs and SDS-AgNPs as control experiments. The solution of SDS-AgNPs underwent vivid color change for many metal ions (Fig. 2b), like Ni2+, Cd2+, Pb2+, Co2+, Mn2+, and Cu2+, which is attributed to be electrostatic interaction resulting in very poor selectivity. AMP-AgNPs showed a similar color change as those of AMP-SDS-AgNPs except for Mn2+ (Fig. 2a), which suggests metal ion binds with N atom of AMP. In fact, only the presence of Ni2+ led to the color change of AMP-SDS-AgNPs (Fig. 2c), demonstrating that AgNPs decorated with AMP and SDS exhibited excellent selectivity to Ni2+ over the ones functionalized only by AMP or SDS. This should mainly result from the cooperative effect of electrostatic interaction between Ni2+ and SDS and coordination interaction between Ni2+ and AMP.

Fig. 2
figure 2

a AMP-AgNPs, b SDS-AgNPs, and c AMP-SDS-AgNPs solution in the presence of different metal ions (30 μM)

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Interference analysis of the assay

To determine the specificity of present colorimetric method for detection of Ni2+, we investigated the possible interference of common adulterants. As showed in Fig. 3, the presence of following amounts of foreign species compared with the concentration of Ni2+ resulted in less than ± 10% error: 100 times the concentration of K+, Na+, and Al3+, 60 times the concentration of Ca2+, 50 times the concentration of Co2+, NH4 +, and Cu2+, 30 times the concentration of Cd2+, Mg2+, Ba2+, and Cr2+, 10 times the concentration of Ag+, Mn2+, Zn2+, Fe3+, Fe2+, Cr3+, and Hg2+, and 5 times the concentration of Pb2+. The above results indicate good selectivity of the present colorimetric assay for Ni2+ over potentially interfering compounds.

Fig. 3
figure 3

Selectivity of the AMP-SDS-AgNPs for Ni2+. () bars represent the extinction ratio of AMP-SDS-AgNPs in the presence of Ni2+ or several other competing ions (30 μM), and () bars are the response from mixture of Ni2+ (30 μM) and maximum tolerant amount of competing species.

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Sensitivity of our assay for Ni2+

Under the above optimized conditions, in order to establish AMP-SDS-AgNPs as a sensing probe, we developed analytical assay for the quantification of Ni2+ in aqueous solution. To this, different concentrations of Ni2+ were added individually into AMP-SDS-AgNPs and corresponding UV-vis absorption spectra were measured. As noticed in Fig. 4a, upon incremental addition of Ni2+ to AMP-SDS-AgNPs in aqueous solution, the SPR absorption peak at 396 nm decreased with the appearance of a new diffident peak at 515 nm. It was observed that by increasing the concentration of from 4.0 to 60 μM, the absorbance ratio (A515nm/A396nm) linearly increased as shown in Fig. 4b. This indicated that the aggregation of the AMP-SDS-AgNPs was directly related to the Ni2+ concentration. When the absorbance ratios (A515nm/A396nm) were plotted against the concentrations of Ni2+, a linear correlation coefficient (R) of 0.997 (regression equation of y = 0.012 × + 0.414) was obtained within the concentration range of Ni2+ from 4.0 to 60 μM. The limit of detection (LOD) for Ni2+ was 0.6 μM, which was much lower than the maximum contaminant level of Ni2+ (45 μM) in drinking water by the US Environmental Protection Agency (EPA).

Fig. 4
figure 4

a Absorption spectra of the AMP-SDS-AgNPs in the presence of Ni2+ with various concentrations at 0, 5.0, 8.0, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, and 60 μM, respectively. Inset is the corresponding photograph. b The plot of absorbance ratio (A515nm/A396nm) of AMP-SDS-AgNPs versus the concentration of Ni2+

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Practical application

In order to validate the reliability of our method, lake water and tap water samples were evaluated for Ni2+ using the aforementioned methodology. In addition, recovery experiments using spiked real samples with three different concentrations of Ni2+ were performed and are summarized in Table 1. The results indicate good recoveries in the range of 98.4 to 106%. Average recovery value of 99% was obtained, indicating that the colorimetric assay can be used for the detection of Ni2+ in real samples from different fields with high accuracy.

Table 1 Recovery test results of the assay in tap and lake water samples with spiked Ni2+
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Conclusions

In summary, we have developed a colorimetric probe for Ni2+ based on the aggregation of AMP- and SDS-cofunctionalized AgNPs. The functionalized AgNPs for colorimetric sensing of Ni2+ exhibited much higher selectivity over the counterparts modified only with AMP or SDS. This is attributed to be electrostatic interaction between Ni2+ and SDS, and Ni2+ binds with N atom of AMP. Thus, AMP- and SDS-functionalized AgNPs provide an effective pathway for rapid, sensitive, selective, and visual detection of Ni2+ in the presence of other metal ions.