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.
This article describes the glutathione-triggered disassembly of gold nanocomposites composed of gold cores and water-soluble copolymers [poly(N-n-isopropylacrylamide-co-acryloyldiethyletriamine)] attached to the surfaces of gold cores. The gold nanocomposites exhibit a bluish purple color because of the assembled gold cores that are conjugated with the diethylenetriamine groups incorporated into the copolymers. Glutathione added to the gold nanocomposite solution adsorbs onto the surface of the gold cores to liberate diethylenetriamine groups, resulting in spontaneous disassembly that changes the color of the solution to a reddish shade. Increasing the glutathione concentration facilitates the spontaneous disassembly of the gold nanocomposites. For the determination of glutathione, the colorimetric change of the gold nanoparticles is quantified with the a* value of the L*a*b* color coordinates defined by the CIE (Commission Internationale de l'Eclairage) chromaticity diagram. A linear relationship between the a* value and the glutathione concentration of up to 6 x 10(-6) mol/L is obtained 15 min after the addition of glutathione that has a detection limit (defined as 3sigma) of 2.9 x 10(-8) mol/L. The colorimetric assay is successfully applied to the determination of glutathione in eye drops and health supplements.
Biological sensing
technology utilizing nanoparticles extends through
a diverse range of fields. The nanosensing is controlled using the
assembly/disassembly of nanoparticles dominated by interaction forces
between them. Although the interaction potential surface gives decisive
information on the sensing mechanism, evaluating the quantitative
profile has been impossible due to extremely complicated interactions
of conjugated soft matter. In this study, a model-potential-free determination
of the interaction potential surfaces was devised by combining small-angle
scattering and liquid-state theory. The model-potential-free liquid
theory was developed for colloidal nanoparticles inherently with strong
van der Waals attraction forces by their nanoscopic size. The present
method extracts interaction potential between nanoparticles even in
systems with complicated interactions due to conjugated soft matter.
By applying this determination method to a glutathione-triggered biosensing
reaction, interaction potential curves between biosensing nanoparticles
were realized for the first time. The analysis revealed peculiar potential
surfaces of the sensing nanoparticles. The mechanism of colorimetric
nanosensing function based on surface plasmon resonance is discussed
from the viewpoint of the assembly/disassembly of nanoparticles in
nanocomposites dominated by the interaction potential surfaces.
The nano gap and the interaction aspect between Raman scattering enhancing nanoparticles conjugated with a functional copolymer were revealed based on the determined interaction potential surface.
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