Yaniv
Ben-Amram
,
Ran
Tel-Vered
,
Michael
Riskin
,
Zhen-Gang
Wang
and
Itamar
Willner
*
Institute of Chemistry, Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel. E-mail: willnea@vms.huji.ac.il; Fax: +972-2-6527715; Tel: +972-2-6585272
First published on 13th October 2011
Au nanoparticles (NPs) functionalized with electropolymerizable thioaniline units and with dithiothreitol ligands were synthesized. The NPs were electropolymerized onto Au-coated glass surfaces in the presence of the alkaline-earth metal ions Mg2+, Ca2+, Sr2+ or Ba2+, to yield the respective ion-imprinted bis-aniline-bridged Au NPs composites on the Au surfaces. After elimination of the ions from the crosslinked matrices, specific imprinted ion recognition sites were generated in the composites. Selective association of the respective ions to the imprinted sites consisting of the dithiothreitol ligands is demonstrated. The high affinities of the metal ions to the respective imprinted sites lead to impressive sensitivities (fM range). The association of the ions to the imprinted sites is monitored by surface plasmon resonance spectroscopy, and the coupling between the localized plasmon of the NPs and the surface plasmon wave is used as an amplification mechanism.
The imprinting of specific recognition sites for molecular substrates, ions or anions in organic or inorganic polymer matrices has been extensively studied.12 For example, polymerizable 1,3-diketone ligand units were used as templates for the association of Ca2+ ions.13Copolymerization of the complexes consisting of N,N′-dimethyl-N,N′-bis-(4-vinylphenyl)-3-oxapentanediamide monomers, under crosslinking conditions, yielded, after the removal of the calcium ions, imprinted polymer matrices for the selective binding of Ca2+. The matrix was then used for the chromatographic separation of Ca2+ from Mg2+ ions. Also, different thiolated ligands were used for the electrochemical14 or optical15–17detection of metal ions through the modification of Au electrodes or Au NPs. For example, electrochemical detection of alkaline-earth metal ions by dihydroxy-thiolated ligands associated with a Au surface was demonstrated using impedance spectroscopy.14 Also, the aggregation of crown ether-modified Au NPs in the presence of alkali-metal ions or alkaline-earth metal ions, with the accompanying red-to-blue color transition of the nanoparticles, was used for the colorimetric detection of the ions. Similarly, the aggregation of phenanthroline-modified Au NPs in the presence of Li+ ions was used for the colorimetric detection of the ions.15–17 In this study, we implemented the dithiothreitol ligand to imprint specific recognition sites for the alkaline-earth ions Mg2+, Ca2+, Sr2+ and Ba2+ in electropolymerized Au NPs composites crosslinked by bis-aniline bridging units, and associated with Au surfaces.
![]() | ||
Scheme 1 Schematic presentation for the electropolymerization of a bis-aniline-crosslinked Au NPs composite on a Au-coated electrode for the sensing of Mg2+, Ca2+, Sr2+ or Ba2+ using dithiothreitol as a ligand for the imprinted metal ions. |
![]() | ||
Fig. 1 (A) SPR curves corresponding to: (a) the thioaniline-modified Au surface before electropolymerization; (b) the bis-aniline-crosslinked Au NPs composite electropolymerized on the Au surface in the presence of Mg2+, 10 mM; and (c) the Mg2+-imprinted bis-aniline-crosslinked Au NPs matrix, following the removal of the imprint ion. All measurements were performed in triply-deionized water. (B) SPR curves corresponding to the Mg2+-imprinted bis-aniline-crosslinked Au NPs composite: (a) before, and (b) after the addition of Mg2+, 1 nM. (C) SPR curves corresponding to the non-imprinted bis-aniline-crosslinked Au NPs composite (a) before, and (b) after the addition of Mg2+, 1 nM (the non-imprinted measurements were performed in a 10 mM tetrabutylammonium acetate solution, pH = 6.2). |
Fig. 2A, curve (a), shows the sensogram (reflectance changes at a fixed angle of θ = 63.5°) of the Mg2+-imprinted matrix upon interaction with variable concentrations of Mg2+ ions. As the concentration of Mg2+ ions increases, the reflectance changes are intensified. Fig. 2B, curve (a), depicts the resulting calibration curve. An impressive sensitivity for analyzing Mg2+ ions is observed with a linear response in the range of 20–100 fM, Fig. 2B, inset. The reflectance changes level off to a saturation value at a concentration of Mg2+ ions corresponding to ca. 200 pM. Fig. 2C, curve (b), shows the interaction of the non-imprinted composite with variable concentrations of Mg2+ ions. Evidently, the non-imprinted Au NPs composite does not respond in the concentration range where significant reflectance changes are observed for the imprinted surface, curve (a), and only minute reflectance changes are observed for Mg2+ concentrations up to 50 pM. These results indicate that the non-imprinted Au NPs matrix lacks affinity for binding Mg2+.
![]() | ||
Fig. 2 (A) Sensograms corresponding to the reflectance intensities changes of the Mg2+-imprinted Au NPs matrix, at θ = 63.5°, upon the addition of variable concentrations of: (a) Mg2+: (a1) 20, (a2) 40, (a3) 60, (a4) 80, (a5) 100 fM, and (a6) 2, (a7) 5, (a8) 20, (a9) 50, (a10) 100, (a11) 250 pM. (b) Ca2+: (b1) 80, (b2) 100 fM, and (b3) 2, (b4) 5, (b5) 20, (b6) 50, (b7) 100, (b8) 250, (b9) 500 pM. (c) Sr2+: (c1) 50, (c2) 100, (c3) 250, (c4) 500 pM, and (c5) 1, (c6) 5 nM. (d) Ba2+: (d1) 50, (d2) 100, (d3) 250 pM, and (d4) 1, (d5) 5, (d6) 50 nM. The inset shows a magnification of one concentration of the sample analyzed. (B) Calibration curves relating the reflectance changes to the concentrations of (a) Mg2+, (b) Ca2+, (c) Sr2+ and (d) Ba2+, on the Mg2+-imprinted matrix. The inset shows the lower concentration region of the Mg2+ calibration curve. (C) Calibration curves relating the reflectance changes to the concentrations of Mg2+ on: (a) the Mg2+-imprinted, and (b) the non-imprinted matrices (the non-imprinted electropolymerization was performed in a 10 mM tetrabutylammonium acetate solution, pH = 6.2). Reflectance changes values presented in the calibration curves were taken after 6 min of measurement. All measurements were performed in triply-deionized water. Error bars correspond to a set of N = 5 measurements. |
A further aspect that was addressed relates to the selectivity of the imprinted sites. Fig. 2A, curves (b), (c) and (d), show the reflectance changes of the Mg2+-imprinted Au NPs composite upon interaction with the alkaline-earth ions Ca2+, Sr2+ and Ba2+, and Fig. 2B depicts the resulting calibration curves. It should be noted that the reflectance changes, upon each addition of the Mg2+ ions, were recorded for 10 min. Within this interval, the reflectance reached a saturation value (see Fig. 2(A) inset, demonstrating the expansion of one of the measurements). This implies that the addition of the Mg2+ ions was performed after the reflectance reached a constant value. Furthermore, one may realize that after ca. 6 min, the reflectance changes reached ca. 85% of the saturation value, and this time-interval was used in the extraction of the calibration curves. The time-dependent increase in the reflectance change is presumably controlled by the diffusion of the ions to the imprinted sites. Clearly, the Sr2+ and Ba2+ ions are not sensed by the Mg2+-imprinted Au NPs composite, while Ca2+ ions reveal only low affinity for the matrix, as reflected by the substantially lower reflectance changes as compared to Mg2+. The ultrasensitive detection of the ions by means of SPR spectroscopy is attributed to the coupling between the localized plasmon associated with the Au particles and the surface plasmon wave. The changes in the dielectric properties of the Au NPs matrix and eventually, the changes in the inter-particle distances, due to binding of the ions, alter the coupling phenomenon between the localized plasmon and the surface plasmon wave. This leads to the SPR spectral shifts that allow the sensitive detection of Mg2+ ions. Realizing that the ultrasensitive detection of Mg2+ ions was achieved by the Mg2+-imprinted matrix, we examined the possibility to imprint ionic recognition sites for the other alkaline-earth ions: Ca2+, Sr2+ and Ba2+.
The goal of this study was to evaluate the sensitivity and selectivity, which are achieved by the imprinting process. Accordingly, imprinted Au NPs composites for each of the ions were prepared by the electropolymerization of the functionalized Au NPs in the presence of the different ions, following Scheme 1.
We find that all imprinted matrices exhibit high reflectance changes upon the interactions with the imprinted ions, while the non-imprinted composites do not exhibit significant reflectance changes (see Fig. S2, ESI†). Furthermore, we find that the imprinting process leads to selectivity of the resulting sensing matrices. Fig. 3A shows the calibration curves corresponding to the analysis of the different ions by the Ca2+-imprinted Au NPs composite. The reflectance changes for analyzing Ca2+ by the Ca2+-imprinted matrix are high. In the concentration range of Ca2+ of 40–100 fM, a linear dependence between the reflectance changes and the concentration of Ca2+ ions is observed, and the reflectance changes level off to a saturation value at a concentration of Ca2+ ions that corresponds to ca. 100 pM. The interaction of the Ca2+-imprinted Au NPs composite with Mg2+ ions leads to substantially lower reflectance changes, Fig. 3A, curve (b), implying lower affinity for the binding of Mg2+ to the Ca2+-imprinted sites. Similarly, we find that the Ca2+-imprinted matrix reveals little affinity for the association of Sr2+ or Ba2+ ions, as reflected by the low reflectance changes at the entire concentration range (for the respective sensograms, see Fig. S3, ESI†). The experimental results reveal two important conclusions: (i) the imprinted recognition sites for Mg2+ or Ca2+ in the Au NPs composites led to ultrasensitive matrices for the sensing of the imprinted ions. (ii) The Mg2+- and Ca2+-imprinted matrices demonstrate selectivity, and Sr2+ and Ba2+ can be discriminated by these matrices. It should be noted that the observed selectivity is affected by the formation of sterically imperfect cavities for the association of the two similar sized ions.
![]() | ||
Fig. 3 (A) Calibration curves relating the reflectance changes to the concentrations of: (a) Ca2+, (b) Mg2+, (c) Ba2+ and (d) Sr2+ on the Ca2+-imprinted matrix. The inset shows the lower concentration region of the Ca2+ calibration curve. (B) Calibration curves relating the reflectance changes to the concentrations of: (a) Sr2+, (b) Ca2+, (c) Mg2+ and (d) Ba2+ on the Sr2+-imprinted matrix. The inset shows the lower concentration region of the Sr2+ calibration curve. (C) Calibration curves relating the reflectance changes to the concentrations of: (a) Ba2+, (b) Ca2+, (c) Sr2+ and (d) Mg2+ on the Ba2+-imprinted matrix. The inset shows the lower concentration region of the Ba2+ calibration curve. All measurements were performed in triply-deionized water. Error bars correspond to a set of N = 5 measurements. |
Significantly improved selectivity is demonstrated for Sr2+ or Ba2+-imprinted Au NPs composites. The calibration curves corresponding to the reflectance changes of the Sr2+ and the Ba2+-imprinted matrices upon analyzing the different ions are shown in Fig. 3B and C, respectively (the sensograms corresponding to the analyses of the respective ions are depicted in Fig. S4 and Fig. S5, ESI†). For the Sr2+-imprinted Au NPs composite, high reflectance changes in the Sr2+ concentration range of 20–100 fM are observed, whereas in this concentration range the other ions do not show any significantly detectable values. The calibration curve for analyzing Sr2+ by the Sr2+-imprinted matrix levels off to a saturation value at a Sr2+ concentration of ca. 200 pM. The other ions, Ca2+, Mg2+ and Ba2+ show lower reflectance changes up to a concentration of 1 nM. Similarly, the Ba2+-imprinted Au NPs matrix reveals impressive sensitivity and selectivity for the detection of Ba2+ ions, Fig. 3C. The reflectance changes reach a saturation value at 25 pM of Ba2+. Within this concentration range, the other ions Mg2+, Ca2+ and Sr2+ yield only minute reflectance changes, and these remain very low even at higher concentrations. It should be noted that the dynamic range for analyzing the different metal ions by the imprinted Au NPs matrices is 20–100 fM. At higher concentrations, the readout signals level off to a saturation value, due to the saturation of the imprinted sites by the respective metal ions. Thus, a possibility to increase the dynamic range for the sensing of the ions would involve the increase in the population of the imprinted sites. This might be accomplished by either increasing the roughness of the support, or by increasing the thickness of the sensing matrix.
The different ion-imprinted Au NPs composites revealed an unaffected sensing performance for at least six days. Furthermore, we find that the sensing matrices can be regenerated by elimination of the ions bound to the imprinted sites by rinsing the composite with an acidic aqueous solution, pH = 1.5, followed by washing the composite with distilled water. In this context, it is worthwhile to address the mechanism of the binding of the ions to the imprinted sites and the ability to sense a specific imprinted ion in the presence of an excess of foreign ions. We find that the regeneration of the sensing matrices and the elimination of the ions bound to the imprinted sites can be achieved by rinsing the composite with pure distilled water (see Fig. S6, ESI†). This suggests that the association of the ions and their dissociation to and from the imprinted sites, are reversible processes. It should be noted, however, that the regeneration of the sensing matrices with pure water is substantially slower in comparison to using an acidic aqueous solution, pH = 1.5. This may be attributed to the protonation of the dithiothreitol ligands, a process that facilitates the elimination of the ions. The reversibility of the binding of the ions to the matrices, and the ability to detect the specific ion in the presence of an excess of foreign ions, were demonstrated by analyzing Ba2+ ions by the Ba2+-imprinted Au NPs matrix, in the presence of an excess of Mg2+, Ca2+ and Sr2+ ions, Fig. 4. In this experiment, the Ba2+-imprinted Au NPs matrix was subjected to a mixture of the ions Mg2+, Ca2+, Sr2+, each 10 pM, which resulted in a reflectance change of ca. ΔR = 70 a.u. Subsequently, the matrix was treated with different concentrations of Ba2+. This resulted in a maximum total reflectance change of ca. ΔR = 200 a.u. These results suggest that Ba2+ ions at a concentration of 60 fM can be sensed at a background signal of Mg2+, Ca2+ and Sr2+ at a concentration of 10 pM each. Furthermore, upon subjecting the surface first to Ba2+ ions, 3 pM, a total reflectance change of ca. ΔR = 200 a.u. was observed, and the subsequent stepwise addition of each of the ions Mg2+, Ca2+, Sr2+, each 10 pM, or as a mixture of the ions (each 10 pM), did not affect the reflectance change of the system. Also, analyzing a mixture of Ba2+ ions, 3 pM, and Mg2+, Ca2+, Sr2+, each 10 pM, resulted in a reflectance change of ca. ΔR = 200 a.u. These results indicate that Ba2+, at a concentration of 3 pM, saturates the imprinted sites and excludes the binding of the other ions that exhibit lower affinity for the sites. On the other hand, the primary binding of the ions exhibiting low affinity to the Ba2+-imprinted sites is followed by an exchange process that saturates the matrix with the Ba2+ ions (exhibiting high affinity towards the imprinted sites). The fact that the sensogram corresponding to the sensing of Ba2+ ions saturates at a reflectance change value of ΔR = 200 a.u. at an added concentration of 3 pM, implies that the Mg2+, Sr2+ and Ca2+ ions were exchanged by the Ba2+ ions, exhibiting higher affinity to the imprinted sites. Without such an exchange mechanism, and in the presence of the Mg2+, Sr2+ and the Ca2+ ions, the reflectance changes would have reached the saturation value at a Ba2+ concentration corresponding to ca. 80 fM.
![]() | ||
Fig. 4 Sensogram corresponding to the reflectance intensities changes of the Ba2+-imprinted Au NPs matrix, at θ = 63.5°, upon the addition of: (a) Mg2+, Ca2+ and Sr2+, 10 pM, each, and upon the subsequent addition of Ba2+ ions: (b1) 60, (b2) 600 fM, and (b3) 1.2, (b4) 2, (b5) 3 pM. All measurements were performed in triply-deionized water. |
Footnote |
† Electronic supplementary information (ESI) available: An AFM image showing the morphology of the electropolymerized Au aggregated composite, calibration curves relating the reflectance changes to the concentrations of the different ions on the imprinted and non-imprinted matrices, and sensograms corresponding to the changes in the reflectance intensities of the metal ions-imprinted Au NPs matrices, upon the addition of variable concentrations of the metal-ion analytes. See DOI: 10.1039/c1sc00403d |
This journal is © The Royal Society of Chemistry 2012 |