Catalytic Growth of Au Nanoparticles by NAD(P)H Cofactors: Optical Sensors for NAD(P)⁺‐Dependent Biocatalyzed Transformations

Abstract

Increasing efforts are directed to the application of metal and semiconductor nanoparticles (NPs) for the development of electronic or optical sensory systems.[1] Metal or semiconductor NPs functionalized with nucleic acids were employed as amplifying labels for the detection of DNA; the dissolution of the nanoparticles was used to follow DNA hybridization events.[2] Also, charge injection from semiconductor nanoparticles into electrodes and the generation of photocurrents was used to follow hybridization processes [3] and biocatalytic transformations.[4] The catalytic deposition of metals onto metal nanoparticles conjugated to DNA-hybridized complexes on surfaces was used as a sensor for DNA through conductivity [5] or microgravimetric quartz crystal microbalance [6] measurements. The optical detection of processes in the presence of metal and semiconductor NPs has become a common practice in analysis. Besides the use of semiconductor quantum dots as fluorescence labels in sensors, [7] the fluorescence quenching of semiconductor quantum dots has been employed in different sensing paths.[8] The plasmon absorbance of metal nanoparticles, such as Au NPs, and specifically the interparticlecoupled plasmon absorbance of aggregated NPs was extensively used to follow molecular [9] and biomolecular [10] recognition processes. The use of semiconductor or metallic NPs as probes to follow biocatalytic processes is less established, with only a few reports for these applications. [4,11] Nicotinamide adenine dinucleotide (NAD + )-and nicotinamide adenine dinucleotide phosphate (NADP + )-dependent enzymes are important in biocatalyzed synthesis.[12] Extensive efforts have been directed towards the development of electrochemical sensors based on NAD(P) + -dependent enzymes.[13] Herein, we report the catalyzed growth of gold nanoparticles in the presence of NAD(P)H cofactors. We apply the process to the quantitative optical analysis of NAD(P)H cofactors and to the analysis of NAD(P) + -dependent biocatalyzed reactions in solutions and on surfaces.The solution for the growth of the particles consisted of citrate-stabilized Au NPs (4.0 10 À10 m in 13 nm AE 1-nm particles), HAuCl 4 (1.8 10 À4 m), and CTAB (7.4 10 À2 m) as a surfactant. Figure 1 shows the changes in the UV/Vis spectra of the growth solution upon interaction with different concentrations of NADH. In the absence of NADH, the solution displays an absorbance band at l = 392 nm, characteristic of the AuCl 4 À component (Figure 1, curve a). Upon addition of NADH, this band disappears instantaneously and the characteristic orange color of the system is depleted (curve b), and then the slow buildup of the absorbance of the particle plasmon is observed. As the concentration of NADH increases, the absorbance of the Au particles increases and is shifted to longer wavelengths (from 523 to 530 nm; Figure 1, curves c-h). The inset in Figure 1 shows the calibration curve derived from the changes in the absorbance at l = 524 nm as the concentration of NADH increases. Figure 2 shows...

Gold Nanoparticle-Based Colorimetric Sensors for Detection of DNA and Small Molecules

“…24,25,[33][34][35][36] To begin, small AuNPs are prepared as seeds. 10 Reducing agents such as hydrogen peroxide, 34 hydroxylamine, 23,33 NADH, 22 flavonoids, 35 ascorbic acid, 25 hydroquinone 24 or neurotransmitters (e.g., dopamine) 37 are then added along with AuCl 4 -salt in the presence of a surfactant-typically CTAB 22 or cetyltrimethylammonium chloride (CTAC) 24,34 to promote growth of the AuNP seeds.…”

“…The 11 extent of the enlargement of the gold seeds and the accompanying shift in optical absorbance is typically proportional to the amount of reducing reagent used. Catalytic growth of AuNP seeds has been coupled with several analytical methods such as absorption spectroscopy, 22,34,[37][38][39] resonance light scattering, 40 fluorescence spectroscopy, 41 and electrochemistry 24,42 in order to improve assay sensitivity. For example, electrocatalytic growth of AuNPs on an electrode surface greatly facilitated electron transfer and improved the sensitivity of the NADH detection.…”

“…Lactate/ethanol concentration can also be detected by NADH produced during the enlargement of AuNPs. 22 These biosensing protocols mainly rely on the reduction of AuCl 4 -by enzymatically-produced hydrogen peroxide or NADH as reducing agents to achieve AuNP enlargement and thereby generate a visible color readout. wavelengths with the decreased absorbance, and a change in solution color was observed as well, indicating that I − contributed to signal amplification.…”

“…Colorimetric detection of NADH has been reported based on anti-aggregation of AuNPs via the self-dehydration condensation of boronic acids and boronic acid-diol binding. 100 One recent report described the optical detection of NADH based on AuNP growth via the strong reduction of Au 3+ to Au 0 by NADH, 22 which alters the optical properties of the AuNPs. A similar assay employed reduction of Cu 2+ to Cu 0 by NADH, resulting in the formation of a core-shell structure through the deposition of Cu 0 on the 20 AuNP surface.…”

“…Specifically, the enlargement of AuNPs can be achieved by silver enhancement, 245 or seed-mediated growth under mild reducing conditions in a growth solution which contains gold precursor and surfactant. 22 The of AuNRs, generating a blue shift in SPR absorption and a visible color change from bluish-green or red to light red and then to colorless as the target concentration increased. 59 However, incubation for more than 5 hours at high temperature (50-70 o C) is required to observe the color change, and so far only inorganic targets have been used with these sensor platforms.…”