Blue-to-Red Chromatic Sensor Composed of Gold Nanoparticles Conjugated with Thermoresponsive Copolymer for Thiol Sensing

Abstract

We describe the first determination of thiol compounds with gold nanocomposites composed of gold nanoparticles and thermoresponsive copolymers having polyamino groups. The gold nanocomposites, which are used as a chromatic sensor, reveal chromatic change from blue to red with thermal stimuli, heating followed by cooling the solution. The blue-to-red chromatic change results from disassembly of the gold nanocomposites, which arises from shrinkage of the thermoresponsive copolymers bound to the gold nanoparticle surfaces due to the phase transition induced by thermal stimuli. The disassembly is inhibited by addition of thiol compounds through displacement of the adhered thermoresponsive copolymers. The detached copolymers no longer influence morphological change of the gold nanocomposites. Corresponding with increase of concentration of the thiol compounds, a solution of the gold nanocomposites after the thermal stimuli shows chromatic change, which was quantified with the a* value in L*a*b* chromatic coordinates. A linear relationship between the a* value and concentration of cysteine, examined as a bio-important thiol, is obtained below 7x10(-6) mol dm(-3), estimating a detection limit defined as 3sigma of the blank to be 2.8x10(-7) mol dm(-3). The chromatic sensor of the gold nanocomposites is applied to the determination of cysteine in commercial supplements containing ascorbic acid, which seriously interferes with redox-based determination of cysteine. Analytical results obtained with the chromatic sensor are identical to those obtained with HPLC.

Polymer-functionalized Gold Nanoparticles as Versatile Sensing Materials

“…The present author's group has developed a unique colorimetric sensor using gold nanocomposites conjugated with thermoresponsive copolymers possessing poly(ethyleneamine). 76 …”

Colorimetric Assay of Aminopeptidase N Activity Based on Inhibition of the Disassembly of Gold Nano-composites Conjugated with a Thermo-responsive Copolymer

“…32 The thermo-responsive copolymer poly(Nisopropylacrylamide-co-N-acryloyltriethylenetetramine) (following poly(NIP0.9-TETA0.1)), having a 0.1 molar ratio of Nacryloyltriethylenetetramine and a 0.9 molar ratio of isopropylacrylamide, was copolymerized according to a previous paper. 31 The average molecular weight (Mn) was estimated to be ca. 1.1 ¥ 10 4 by gel permeation chromatography with calibrated poly(styrene) standards.…”

“…The change in the solution color from blue to red was used in a colorimetric assay. 31 Sulfhydryl compounds influence the disassembly of the gold nano-composites significantly. Although cysteine inhibits the disassembly, cysteinylglycine gives no inhibition against the disassembly.…”

“…The inhibition of disassembly results from a replacement of the TETA groups in thermo-responsive copolymers conjugated with gold nano-composites by sulfhydryl compounds through the formation of covalent bonds between a thiol group in the sulfhydryl compounds and the gold surface. 31 The replacement liberates the conjugated thermo-responsive copolymer from the surface of AuNPs. As a result, the liberated thermo-responsive copolymers no longer expand the interparticle distance of AuNPs, when the gold nano-composites solution is heated above the phase-transition temperature.…”

“…We have already reported on the disassembly of aggregated gold nano-composites consisting of AuNPs and a thermo-responsive copolymer having poly(ethyleneamine) groups, and have exploited a blue-to-red colorimetric nano-particles sensor based on disassembly. 31 The disassembly of aggregated gold nanocomposites is provoked by thermal stimuli, heating followed by cooling a solution, which induces a phase transition of the thermo-responsive copolymer conjugated with the AuNPs surface. Since some sulfhydryl compounds, such as cysteine and homocysteine, inhibit disassembly, a gold nano-composites solution develops a colorimetric gradation from red to blue purple in accordance with an increase in the concentration of the sulfhydryl compounds.…”