No abstract

MicroRNA (miRNA) is a promising new type of biomarkers but at a low fM level and hard to be analyzed. Herein proposed is an innovated surface plasmon resonance imaging (SPRi) method merged with a novel in-plane and vertical signal amplification strategy, that is, orthogonal signal amplification to enable a direct determination of sub-fM miRNA-15a (a multiple tumor diagnostic biomarker). The core idea is to add more mass on a target sample spot first along the surficial direction, then upward from the surface. In detection of miRNA, this was realized by coupling a miRNA-initiated surficial cyclic DNA-DNA hybridization reaction with a DNA-initiated upward cyclic polymerization reaction. A perfect SPRi sensing chip with isolated gold islands bordered by hydrophobic CYTOP was fabricated and used to obtain high-quality chip with low fabrication difficulty. As a result, SPRi contrast largely increases, able to reach a limit of detection and limit of quantification down to 0.56 and 5fM for miRNA-15a, about 10-fold improvement of sensitivity compared with a common SPRi detection. The method could quantify standard miRNA-15a spiked in human serum with an ideal recovery ranging from 98.6% to 104.9% and was validated to be applicable to the direct determination of miRNA-15a in healthy and cancer human serums. The orderly and controllable in situ sensitizing strategy is powerful and readily extendable to detection of other miRNAs.

A stepwise strategy was proposed to controllably amplify surface plasmon resonance imaging (SPRi) signals and used to establish a method for sensitive detection of small saccharides and large glycoconjugates. The key is to enlarge the target analytes step by step through a cyclic recognition reaction of concanavalin A (ConA) with dextran, which can easily be integrated into SPRi detection. The reaction is theoretically expected to proceed with infinite cycles by addition of ConA and dextran one after another, and in practice, it allows for performing signal amplification for up to 20 steps, which nearly reaches the limit of the propagation depth of surface plasmon waves. Because of the nonspecific adsorption effect, the maximum signals of small sugars were measured at steps 5-7 by the use of small molecules like ethanol amine to block the nonspecific adsorption. The real applicability of the method was validated by SPRi determination of either immobilized saccharides and glycoproteins or captured carcinoembryonic antigen (CEA) from patient samples with a limit of detection down to 2.5 μM glucose or 50 pg/mL of CEA. A wide extendibility of the method was confirmed by detecting other ConA-recognizable analytes like human IgG, ovalbumin, α-fetoprotein, and analytes convertible to ConA- or dextran-recognizable forms. Notably, other cyclic reactions specific to saccharides or other analytes are in theory exploitable, which will further widen the applicability of the method and strategy.

No abstract