Integrating colorimetry with surface sensitive transducers: Advancing molecular diagnostics in biofluids

Abstract

Colorimetric sensors are preferred for visually detecting target analytes, but achieving ultra-low detection capabilities for food analysis, environmental samples, and molecular diagnostics is challenging. In this study, we interfaced a purpald-based formaldehyde color sensor (as a model probe reaction) with a surface plasmon gold microarray imaging chip. Such hyphenated surface-sensitive signal-amplifying techniques enabled ultra-low detection in biofluids with a linear range of 10−5 – 0.5 ppm compared to the spectrophotometric narrower linear range of 0.01–1 ppm. The detection limit of 86×10−6 ppm from the hyphenated sensing is 140-fold lower than the detection limit from the spectrophotometric method alone (12×10−3 ppm). The absorbance maximum of the purpald-formaldehyde adduct was 553 nm, with no notable tail overlap between the adduct absorbance band beyond 700 nm and the fixed angle source incident wavelength of 800 nm from the surface plasmon instrument. Hence, the possible adduct thiol-surface plasmon gold surface interaction, affinity, and the higher mass and refractive index changes of the adduct are likely causes of the observed signal amplification. We achieved a formaldehyde sample recovery of 92 % from a diluted serum (a minimally invasive assay method), which was comparable to the results in a buffer solution. Further, we extended to demonstrate in diluted urine sample matrices to be noninvasive bioanalysis and determined the interference from urea that could also form a purpald adduct like formaldehyde. In conclusion, coupling a surface-sensitive transducer with less sensitive colorimetric reactions is promising for molecular diagnostics in real samples and can be broadly extended to any other desired molecular targets and sample matrices.