A novel continuous flow (CF) electroless (EL) plating method deposits gold (Au) films which exhibit increased continuity and uniformity relative to batch immersion as measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Transmission UV−vis (T-UV) spectra exhibit lower peak heights (PH) and narrower full-width-half-maxima (fwhm), consistent with microscopy measures. Nanoparticles (NPs) obtained by thermal transformation of CF-EL Au film are larger and monodisperse relative to I-EL NPs. Their extinction spectra exhibit higher PH due to enhanced localized surface plasmon resonance (LSPR) and narrower fwhm than NP from I-EL films. Au deposition during CF-EL plating is calculated to be 78.3% higher than during I-EL plating for each 30-s interval. Optical features of T-UV [PH, fwhm, and resonant extinction maxima (λmax)] increase in proportion to time-integrated mass transport coefficient and suggest that increased mass transfer rate and steady higher average local Au concentration during CF improve continuity, uniformity, and overall smoothness of CF plated films.

Aqueous, acid solutions containing balanced amounts of a strong reductant (formaldehyde, HCHO) and a strong oxididant (N-bromosuccinimide, NBS) allow the first sensitive spectrophotometric analysis of monovalent gold ion, Au(I), using oxidation of color reagent 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB). This new method enables various oxidation states of Au ion to be quantified by balancing reduction potential in a Au solution. At low reductant levels, NBS oxidizes Au(I), which linearly suppresses subsequent oxidation of TMB by NBS to its blue charge-transfer complex of diamine and diimine to 2.00 mg L(-1) of Au, resulting in reduced color formation. The linear range of Au(I) quantitation was increased substantially relative to existing methods: from 0.005 to 1.00 mg L(-1) (R(2) = 0.988). For this range, the limit of detection was 0.0025 mg L(-1), which is comparable to the best reported spectroscopic method to analyze Au(III). At relatively high reductant levels, Au(I) is reduced to Au(0), then subsequently oxidized from Au(0) to Au(III) by addition of NBS. TMB is oxidized to its blue charge-transfer complex via the reduction of the reoxidized Au(III) to Au(0). Balancing redox conditions of HCHO/NBS at a molar ration of 22.7 allows quantitative measurement of Au(I) across a linear concentration range of 0.05-2.00 mg L(-1) (R(2) = 0.997). This balancing redox condition could allow sensitive, quantitative, spectrophotometric analysis of other metal ions besides Au by targeting the metal ion's reduction potential with an associated redox-sensitive color reagent.