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.

Toward rapid analysis, forecast and discovery of bioactive compounds from herbs by jointly using thin layer chromatography and ratiometric surface-enhanced Raman spectroscopy technique.

Conventional isolation and identification of active compounds from herbs have been extensively reported by using various chromatographic and spectroscopic techniques. However, how to quickly discover new bioactive ingredients from natural sources still remains a challenging task due to the interference of their similar structures or matrices. Here, we present a grand approach for rapid analysis, forecast and discovery of bioactive compounds from herbs based on a hyphenated strategy of thin layer chromatography and ratiometric surface-enhanced Raman spectroscopy. The performance of the hyphenated strategy is first evaluated by analyzing four protoberberine alkaloids, berberine (BER), coptisine (COP), palmatine (PAT) and jatrorrhizine (JAT), from a typical herb Coptidis Rhizoma as an example. It has been demonstrated that this coupling method can identify the four compounds by characteristic peaks at 728, 708, 736 and 732 cm-1, and especially discriminate BER and COP (with similar migration distances) by ratiometric Raman intensity (I708/I728). The corresponding limits of detection are 0.1, 0.05, 0.1 and 0.5 µM, respectively, which are about 1-2 orders of magnitude lower than those of direct observation method under 254 nm UV lamp. Based on these findings, the proposed method further guides forecast and discovery of unknown compounds from traditional Chinese herb Typhonii Rhizoma. Results infer that two trace alkaloids (BER and COP) from the n-butanol extract of Typhonii Rhizoma are found for the first time. Moreover, in vitro experiments manifest that BER can effectively decrease the viability of human glioma U87 cells by inducing cell cycle arrest in a concentration-dependent manner.

Profuse color-evolution-based fluorescent test paper sensor for rapid and visual monitoring of endogenous Cu2+ in human urine.

The fluorescent paper for colorimetric detection of metal ions has been widely fabricated using various sensing probes, but it still remains an elusive task to design a test paper with multicolor variation with target dosages for accurate determination. Herein, we report a profuse color-evolution-based fluorescent test paper sensor for rapid and visual monitoring of Cu2+ in human urine by printing tricolor probe onto filter paper. The tricolor probe consists of blue-emission carbon dots (bCDs), green-emission quantum dots (gQDs) and red-emission quantum dots (rQDs), which is based on the principle that the fluorescence of gQDs and rQDs are simultaneously quenched by Cu2+, whereas the bCDs as the photostable internal standard is insensitive to Cu2+. Upon the addition of different amounts of Cu2+, the ratiometric fluorescence intensity of the tricolor probe continuously varied, leading to color changes from shallow pink to blue with a detection limit of 1.3nM. When the tricolor probe solution was printed onto a sheet of filter paper, as-obtained test paper displayed a more profuse color evolution from shallow pink to light salmon to dark orange to olive drab to dark olive green to slate blue to royal blue and to final dark blue with the increase of Cu2+ concentration compared with dual-color probe-based test paper, and dosage scale as low as 6.0nM was clearly discriminated. The sensing test paper is simple, rapid and inexpensive, and serves as a visual platform for ultrasensitive monitoring of endogenous Cu2+ in human urine.

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.