Microchim Acta DOI 10.1007/s00604-014-1340-4
ORIGINAL PAPER
Calix[4]arene functionalized gold nanoparticles for colorimetric and bare-eye detection of iodide in aqueous media and periodate aided enhancement in sensitivity Debdeep Maity & Madhuri Bhatt & Parimal Paul
Received: 1 May 2014 / Accepted: 11 August 2014 # Springer-Verlag Wien 2014
Abstract A water- soluble p-sulphonatocalix[4]arene was synthesized and anchored onto the surface of gold nanoparticles (AuNPs) in aqueous medium. The conjugate was characterized by IR, UV–Vis and TEM analysis. This material responds to iodide in giving a color change from pink to blue which is easily detectable with bare eyes. While periodate itself does not cause any spectral changes, a substantial spectral change can be seen in the presence of traces of iodide. The lower detection limit for iodide in the absence of periodate is 2.5 μM, which is further lowered to 80 nM in presence of periodate. A mechanistic study revealed that the chemisorption of the ions I− and I3−, formed by the reaction I− and periodate on the surface of AuNPs resulted in spontaneous oxidation of the anions. The electron transfer changes the size and morphology of the nanoparticles and results in the color change. The method is specific for iodide. It was successfully applied to the determination of I− in (spiked) waters and in solutions of iodized edible salt.
Keywords Colorimetric sensor . Bare-eye iodide detection . Calix[4]arene . Gold nanoparticles . Periodate aided sensitivity
Electronic supplementary material The online version of this article (doi:10.1007/s00604-014-1340-4) contains supplementary material, which is available to authorized users. D. Maity : M. Bhatt : P. Paul (*) Analytical Discipline and Centralized Instrument Facility, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364002, India e-mail:
[email protected] M. Bhatt : P. Paul Academy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI, G. B. Marg, Bhavnagar 364002, India
Introduction Among biologically important anions, iodide is most important because it is one of the essential micronutrients for normal human growth [1]. Iodide plays key role for healthy development, especially for pregnant women and children, improper balance of it in human body causes thyroid disease and mental retardation [2]. To take care of iodide deficiency disorders, the World Health Organization (WHO) has recommended using of iodized table salt and it is a common practice in many countries [3]. Therefore, development of suitable method for selective and sensitive detection of iodide and its estimation at low concentration in aqueous media is critically important. For this purpose, a number of instrument based techniques, for example ICP-mass, ion chromatography, electrochemistry etc., have been reported [4–7], however, these techniques are time-consuming, involve multistep sample preparation, specific operating skills and more importantly it requires expensive instruments. Alternatively, efforts have been made to develop colorimetric and fluorogenic methods for selective detection of iodide [8, 9]. Significant progress has been made in this direction, for example, anthracene–thymine based Hgcomplex as fluorogenic receptor detects iodide selectively [10]. However, in many of these detections, solubility of sensor in water, selectivity and sensitivity at low concentration are always a matter of concern. For colorimetric detection, gold nanoparticles (AuNPs) based sensors have received considerable attention because of their high extinction coefficient in the visible region owing to the surface plasmon resonance (SPR) and color-tunable behaviour that depends on the interparticle distance and size of the nanoparticles [11]. AuNPs modified with various receptor molecules have been used as colorimetric sensors for detection of metal ions [12–15], anions [16–20] and biologically important molecules [21, 22]. In presence of specific guest analytes, it leads to a rapid colour change usually from
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red to purple and thus AuNPs based sensors were used for the detection of various analytes. In this article, we report a water soluble calixarene functionalized gold nanoparticles as sensor for iodide. Calixarenes are found to be very attractive because depending on the requirement it can be modified, which give rise to a great variety of derivatives with various functional groups and that provide a preorganized architecture for the assembling of converging binding sites [21, 23]. We prepared water soluble p-sulfonatocalix[4]arene in 1,3-alternate conformation and used to anchor it on to the surface of the AuNPs in aqueous media. This functionalized gold particle (p-SC4-AuNPs) detects iodide in aqueous media with high selectivity and sensitivity with sharp colour change, detectable by bare-eyes. The p-SC4-AuNPs also exhibited significantly enhanced detection limit for I− in presence of IO4−, particularly at low concentrations of iodide. For application purpose, the performance of pSC4-AuNPs for the detection/estimation of iodide in aqueous media as real sample has also been studied. All of these results and mechanistic aspects of detection are presented herein.
Experimental Materials and apparatus Chemicals such as tert-butylphenol, formaldehyde (37–44 %), H2SO4, HAuCl4, and trisodium citrate were purchased from Aldrich (www.sigmaaldrich.com). All tetrabutylammonium salts of the anions were purchased from Alfa Aesar (Johnson Matthey Company; www.alfa.com). All other solvents and reagents used in this study were purchased from S.D. Fine Chemicals (www.sdfine.com). Organic solvents used were analytical grade and were used as received for synthetic purpose. Solvents for spectral studies were freshly purified by standard procedures before use. Elemental analyses (C, H and N) were performed on a model Vario Micro CUBE elemental analyzer. NMR spectra were recorded on a model DPX 200 and Avance II 500 MHz Bruker FT-NMR instruments. Infrared spectra were recorded on a Perkin Elmer Spectrum GX FT-IR system as KBr pellets. Mass spectra were recorded on a Q-Tof microTM LC-MS instrument. The UV–Vis spectra were recorded on a CARY 500 scan Varian spectrophotometer. Transmission electron microscope (TEM) images were recorded on HR-TEM (JEOL), model JEM 2100. The samples were dispersed in ethanol and loaded over lacey carbon coated TEM grid (300 mesh), which were dried under ambient condition and imaged at 200 KV acceleration voltage. The elemental mapping of the samples was done using in-situ EDX-STEM (Oxford Instrument Inc, INCA X-sight). DLS and Zeta potential were measured on a model Nano- ZS90 Malvern instrument. AFM study was carried out under ambient conditions
using scanning probe microscope NT-MDT (Model: Ntegra Aura; Moscow) in semi-contact mode using rectangular cantilever of Si3N4. Synthesis of calix[4]arene derivatives Tert-butyl calix[4]arene (1) and dealkylated calix[4]arene (2) were prepared following the literature procedures [24]. Synthesis of pp-sulphonatocalix[4]arene (3) This compound was synthesised following the modified procedure reported by Shinkai et al. [25]. In a typical procedure, calix[4]arene (1 g) was mixed with 10 mL of concentrated H2SO4 and the resulting solution was then heated at 70 °C for 3 h. An aliquot of this solution was poured into water to check the progress of the reaction. The reaction was considered completed when no water-insoluble material was observed. After cooling, the precipitate was separated by filtration, it was dissolved again in methanol (4 mL) and poured into ethyl acetate, which gave precipitate of pure p-sulphonatocalix[4] arene. Yield: 1.05 g (60 %). 1H NMR δH (D2O, 500 MHz): 7.62 (s, 8H, ArH), 4.06 (s, 8H, Ar-CH2-Ar); 13C NMR δC (D2O, 500 MHz): 151.65, 135.77, 128.17, 126.54 (aromatic carbons), 30.56(Ar-CH2-Ar). ES-MS: m/z (%): 743.42 (100 %), [M−], (calc. 743.70). IR (KBr pellet): 3,319 cm-1, ν(O-H); 1,180 and 1,038 cm-1 ν(S=O, S-O). Elemental analysis calcd (%) for C28H24O16S4: C, 45.16; H, 3.25; S, 17.22; found: C, 44.76; H, 3.36; S, 17.17 %. Synthesis of AuNPs in aqueous media In aqueous media, AuNPs were prepared by reduction of HAuCl4 using citrate anion following the reported method [26]. In a typical procedure, a 400 mL aqueous solution of HAuCl4 (1 mM) was boiled with vigorous stirring in a round bottom flask fitted with a reflux condenser, trisodium citrate (38.8 mM) dissolved in water (40 mL) was then added rapidly into this solution. The resulting reaction mixture was heated under reflux for another 15 min, during which time its color changed from pale yellow to deep red. The solution was then cooled to room temperature while being stirred continuously. It was then filtered through 0.2 μm GNWP Nylon membrane to remove insoluble material/large size particles, if any and was diluted to 500 mL and preserved it in a refrigerator at 4 °C before being used. The AuNPs were characterized on the basis of characteristic UV–Vis band, which appeared at 522 nm. The sizes of the nanoparticles were determined by TEM analysis; it appeared nearly monodisperse with an average particle size of 13.3±1.2 nm. The particle concentration of the solution was determined following the Beer’s law using an extinction coefficient of 1×108 M-1 cm-1 at 520 nm for AuNPs having 13.3 nm diameter [27].
Calixarene capped gold nanoparticles for iodide detection
Functionalization of gold nanoparticle with pp-sulphonato calix[4]arene (4) For functionalization of AuNPs, 1 mL of 1.6 mM aqueous solution of p-SC4 (3) was added into the stock aqueous solution of the AuNPs (~3.2 nM, 25 mL) and stirred for 12 h at room temperature in dark. The reaction mixture was then centrifuged for 10 min at 9,500 rpm and the deposited mass was washed three times with water, separated and then characterized (discussed below). The pallet thus obtained was dissolved in HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer solution maintaining pH 7.0 and this solution was used for the sensing study. UV–Vis, λmax: 522 nm (SPR band), 272 nm (calixarene). IR (KBr pellet) 3,431 cm-1, ν(O-H); 1,165 and 1,021 cm-1 ν(S=O, S-O).
Scheme 1 Route for synthesis of p-sulphonato calix[4] arene and its anchoring onto the surface of the AuNPs
Anion sensing study The stock solution of p-SC4-AuNPs (3.2×10−9 M) was used to study ion-binding property using tetrabutylamonium salts of the anions F−, Cl−, Br−, I−, H2PO4−, ClO4−, NO3−, IO4−, BF4−, CN−, HSO4−, IO3−, C7H6O2−, SCN−, CH3COO−, HSO3−, SO32− and S2O42−. The aqueous solution (2 mL) of each anion (2×10−4 M) was added into the 2 mL solution of pSC4-AuNPs in a 5 mL volumetric flask and the UV–Vis spectra of the resulting solutions were recorded. Significant spectral change and sharp colour change was noted for I−, whereas for other anions no significant change either in colour or in spectra were noted. For I−, spectral changes were further recorded with progressive addition of the anion within the concentration range of 10 μM to 120 μM, maintaining the concentration of p-SC4-AuNPs constant (3 nM).
Results and discussion Characterization of pp-sulphonatocalix[4]arene modified gold nanoparticles
may also be attributed to the centrifugation of the ligandmodified AuNPs, a factor that sometimes affects the size distribution [30]. The morphology of p-SC4-AuNPs was investigated by transmission electron microscope, the image recoded shows highly mono dispersed particles with an average size of 13.3±1.2 nm.
Sensing of anions monitored by UV-visible spectroscopy The interaction of p-SC4-AuNPs with various anions was studied in aqueous media and the recognition event was monitored by UV-visible spectroscopy. Detail of the experimental procedure is described in the Experimental Section. Upon addition of anions, only for I− the intensity of the SPR band at 524 nm is substantially reduced and slightly shifted to 527 nm with the formation of a new broad band centering at 660 nm. Because of this spectral change, a distinct change in colour of the solution from pink to blue, detectable by bare eyes was observed (Fig. 1b). For all other anions used in this study, no significant spectral/color change was noted (Fig. 1a). -
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Synthesis of p-sulphonatocalix[4]arene and its anchoring onto the surface of the AuNPs are shown in Scheme 1. Characterization data for p-SC4 and p-SC4-AuNPs are given in the Experimental Section. The IR spectra of citrate stabilized AuNPs, p-SC4 and p-SC4-AuNPs are shown in Fig. S1 (Electronic Supplementary Material, ESM), the characteristic peaks for SO3− at 1,180 and 1,038 cm−1 found for p-SC4 are shifted to 1,165 and 1,021 cm−1, respectively in p-SC4-AuNPs, suggesting the interaction of SO3− group with the surface of the AuNPs [28]. The UV–Vis spectrum of the AuNPs exhibited the SPR band at 522 nm, which has shifted to 524 nm for pSC4-AuNPs (Fig. S2, ESM), as expected for the surface thiolation of nanoparticles [29]. The slight shift in wavelength
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Wavelength (nm) Fig. 1 a UV–Vis spectral change for p-CS4-AuNPs (1.1 nM) in aqueous media in presence of various anions (100 μM) and b Photographic images of p-CS4-AuNPs (3.3 nM) in presence of various anions (100 μM)
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The interference from other anions was also investigated by recording absorption spectra of p-CS4-AuNPs upon addition of a mixture of anions containing 10 % of I− (10 μM) and 90 % of other anions (90 μM). The ratio of the fraction of change of absorption intensity, ΔA660/A524 at 660 nm, was plotted against the mixture of different anions and the bar diagram thus obtained is shown in Fig. 2. It may be noted that no significant interference from other anions, except IO4−, is observed. In case of IO4−, the fraction of change of absorption intensity (ΔA660/A524) is abruptly enhanced. This indicates that IO4−, in presence of I− plays a significant role in the detection of I−, the phenomenon is discussed in the later section.
Optimum conditions for stability of pp-sulphonatocalix[4] arene modified gold nanoparticle and detection of I− To check the stability of p-CS4-AuNPs in aqueous media with respect to pH, the UV–Vis spectra of the solution in the pH range 3.1–11.6 were recorded (Fig. S3, ESM). From these UV–Vis spectra, it appears that the p-CS4-AuNPs is stable in the pH range 5.1–11.6. The stability is further checked by measuring zeta potential (ζ) of the p-CS4-AuNPs solution as a function of pH and the results are given in Table S1 (ESM). Colloidal particles having higher ζ value are more stable due to the strong electrostatic repulsion between the particles, which prevents them to from aggregation [31]. The data in Table S1 clearly show good stability of p-CS4-AuNPs in the pH range 6.0–11.6; at pH 5.1, it is relatively less stable, which are in close agreement with the observation noted in the UV– Vis study. The UV–Vis spectral changes of p-CS4-AuNPs in the pH range 5.1–11.6 were also recorded upon addition of I− (25 μM) and the ratio of absorbance, A660/A524 with and without addition of I− are compared (Fig. S4, ESM), the data suggest that detection of I− can also be made in the same pH range (5.1–11.6), in which p-CS4-AuNPs exhibits stability before addition of I−.
The time required to attain the maximum interaction/ aggregation (incubation time) is also evaluated by recording UV–Vis spectra as a function of time at a particular concentration of p-CS4-AuNPs. The plot of the ratio of absorption intensity (A660/A524) as a function of time (Fig S5, ESM) suggests that the incubation time is around 20 min. Therefore, after the addition of I−, 20 min incubation time was allowed to collect subsequent data. To determine the lower detection limit (LOD) and to investigate the linear responsive range of p-CS4-AuNPs towards detection of I−, the absorption spectra of the solution upon addition of incremental amount of I− were recorded and the resultant colour change were also noted. The pH of the solution was maintained around 7.2 by adding HEPES buffer (10 mM). The spectral change and the colour developed upon addition of I− are shown in Fig. 3; the plot of the ratio of absorption intensity A660/A524 as a function of the concentration of I− added is shown as inset of Fig. 3b. This plot shows that a considerable spectral change has started between 2.0 and 3.0 μM (elaborated in Fig. S6, ESM), therefore 2.5 μM is considered as LOD. The linearity as a function of concentration of I− was noted in the concentration range of 10 to 80 μM, indicating that the quantification study can be done within this range of I− concentration.
High resolution transmission electron microscopic (TEM) analysis The TEM images of p-CS4-AuNPs were recorded before and after addition of I− (Fig. 4a and b). The image before addition of I− showed highly dispersed nature (Fig. 4a) with average particles size of 13.3±1.2 nm, however after addition of I−
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Fig. 2 Bar diagram showing the ratio of fraction of change in absorption intensity at 660 nm, ΔA660/A524 for p-CS4-AuNPs upon addition of different anions (small bars in grey colour) and a mixture of anions containing 10 % of I− (10 μM) and 90 % of other anions (90 μM, bars in black colour)
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Fig. 3 a Colour change of the p-CS4-AuNPs upon addition of different concentration of I− A-0 μM, B-5 μM, C-10 μM, D-15 μM, E-20 μM, F-25 μM, G-40 μM, H-60 μM; b UV–Vis spectral change of p-CS4AuNPs before and after addition of different concentrations of I− (0 to 100 μM) and (inset of b) plot of the ratio of absorption intensity A660/ A524 as a function of the concentration of I− added
Calixarene capped gold nanoparticles for iodide detection
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Fig. 4 TEM images of a p-CS4-AuNPs and b the same upon addition of I− 25 μM; c DLS measurement for p-CS4-AuNPs before and after incubation of 25 μM of I− ion
(25 μM), the mono dispersed p-CS4-AuNPs became highly aggregated, as may be seen in Fig. 4b. The aggregation is further confirmed from dynamic light scattering (DLS) measurement. Dynamic light scattering study The aggregation of p-CS4-AuNPs in solution was further investigated by dynamic light scattering (DLS) measurement. The DLS data of the p-CS4-AuNPs before addition of I− and after addition of I− (25 μM) were recorded (Fig. 4c). The intensity weighting Gaussian distribution curves showed the average hydrodynamic diameter of well dispersed p-CS4AuNPs was 37 nm and that upon addition of I− increased to 87 nm. This increased diameter is due to aggregation of pCS4-AuNPs.
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Fig. 5 UV–Vis spectrum of p-CS4-AuNPs (3.5 nM) before addition of I− and IO4− (a), after addition of IO4− (100 μM) (b) and after addition of I− (1.0 μM) and IO4− (100 μM) (c); corresponding colour of the solutions are shown as inset. Bar diagram (b) for the plot of the ratio of change in absorbance (ΔA660/A524) as a function of the concentration of I− added (small bars with ash colour) and the same with addition of 100 μM of IO4− (longer bars with black colour) is also shown
the ratio of change in absorption intensity ΔA660/A524 as a function of the concentration of I− in the low concentration region (0 to 9 μM) in absence and in presence of IO4− is shown in Fig. 5b, which clearly showed dramatic enhancement of the ratio of intensities (black bar) in presence of
Periodate aided sensing of iodide at low concentration The aqueous solution of p-CS4-AuNPs detects I−, however the colour and spectral change at low concentration of anion (
