Silver nanoparticles/activated carbon composite as a facile SERS substrate for highly sensitive detection of endogenous formaldehyde in human urine by catalytic reaction.

Most of efforts have been made to prepare high performance surface-enhanced Raman scattering (SERS) substrate for amplifying Raman signals. It still remains a grand challenging task in building a simple, conventional and low-cost SERS substrate with highly dense hotspots for improved sensitivity of the target analytes. Here, we report a very dexterous strategy to fabricate a distinctive SERS substrate with high density hotspots, using common adsorbent activated carbon (AC) as template to assemble silver nanoparticles (Ag NPs). It can be estimated that the enhancement effect of Ag NPs/AC composite is about 6.5-fold that of bare Ag NPs. Different from the resonant dyes, however, formaldehyde (FA) is a Raman-inactive molecule even though enhanced. Considering that, a novel method for quantitative analysis of FA using the Ag NPs/AC composite as SERS sensor has been developed, based on the catalytic effect of trace FA on the oxidation of malachite green (MG) through bromate under acidic condition. The change of MG from reduced form into oxidized leucomalachite green (LMG) results in the quench of Raman signals of MG, responding to 0.07 ppb FA that is about 2 orders of magnitude lower than the limit defined by the Nash's method as a standard procedure recommended in Europe, Japan and China. Moreover, SERS examinations of endogenous FA in human urine signify that the proposed method has high selectivity, reliability and accuracy. Thus, as-fabricated Ag NPs/AC composite is adequate as inexpensive and versatile SERS sensor utilized in the quantification of trace targets in various complicated matrices.

Ehrlich Reaction Evoked Multiple Spectral Resonances and Gold Nanoparticle Hotspots for Raman Detection of Plant Hormone.

Surface-enhanced Raman scattering (SERS) by use of noble metal nanoparticles has become a powerful tool to determine a low-concentration target by unique spectral fingerprints, but it is still limited to the Raman-inactive and nonresonant biomolecules such as amine acids, proteins, and hormones. Here, we report an Ehrlich reaction based derivative strategy in combination with gold nanoparticles (Au NPs) hotspots for the selective detection of indole-like plant hormones by SERS spectroscopy. Ehrlich reaction of p-(dimethylamino)benzaldehyde (PDAB) with the indole ring chemically transformed plant hormone indole-3-butyric acid (IBA) into a Raman-active and resonant derivative with an extended π-conjugated system in the form of a cation, which produced a new absorption band at 626 nm. On the other hand, cationic IBA-PDAB highly evoked the aggregation of Au NPs with negative citrate ligands to form the effective Raman hotspots and gave rise to the new absorption ranging from 600 to 800 nm. Significantly, the spectral overlap among IBA-PDAB, aggregated Au NPs, and the exciting laser initiated the multiple optical resonances to generate the ultrahigh Raman scattering with a sensitive limit of 2.0 nM IBA. The IBA in the whole sprouts and various parts of pea, mungbean, soybean, and black bean has been identified and quantified. The reported method opens a novel avenue for the SERS detection of Raman-inactive analyte by a proper derivation.