Colorimetric plasmonic sensing of formaldehyde by in situ formation of core-shell bimetallic gold-silver nanoparticles in liquid films.

BACKGROUND: Plasmonic sensing approaches involving in-drop enrichment/sensing of volatiles have significantly improved the sensitivity and selectivity of noble metal nanoparticle (NP)-based analytical methods. Colloidal solutions act in these approaches both as acceptor phases and as recognizing elements for optical detection. However, the reduced surface area-to-volume (SA/V) ratio associated to the spherical configuration of NPs-containing microdrops hinders the mass transfer in the acceptor phase and thus results in suboptimal analytical performance. In this sense, the assessment of alternative geometries that could potentially enhance the mass transfer while ensuring ease of operation would be highly desirable.

RESULTS: The present work evaluates the suitability of liquid films of plasmonic NPs for enrichment/colorimetric sensing purposes. For this, the circular section of affordable stainless steel safety pins is employed as holder of stable microvolume films with enhanced SA/V ratio. As a proof of concept, liquid films of spherical gold NPs containing Tollens' reagent have been evaluated as acceptor phases for formaldehyde enrichment and sensing via in situ formation of core-shell bimetallic gold-silver nanoparticles. The performance of the proposed approach has been compared with that of alternative LPME approaches, showing 2.2-3.0-fold higher enrichment factors and improved sensitivity. The method yielded a limit of detection of 56.7 nM and a repeatability of 6.8 % (N = 8) when a portable cuvetteless microvolume UV-vis spectrophotometric system was used, and has been successfully applied to the determination of formaldehyde in different waters and textile samples.

SIGNIFICANCE: The present study represents the first report that evaluates planar liquid films for simultaneous enrichment and optical sensing, and provides an effective and straightforward way to obtain liquid film-like configurations of variable volumes of acceptor phase for improved mass transfer. The proposed approach shows substantial potential for the development of sensors and nanosensors with enhanced sensitivity.