Hg2+ detection sensitivity modulated by ssDNA sequence based binary numbers in bimetal-plasmonic optical fiber sensor that triggers ssDNA chemo-mechanical folding.

This work demonstrates the quantitative assay of cardiac Troponin I (cTnI), one of the key biomarkers for acute cardiovascular diseases (the leading cause of death worldwide) using the fluorescence-based sandwich immune reaction. Surface plasmon coupled emission (SPCE) produced by non-radiative coupling of dye molecules with surface plasmons being excitable via the reverse Kretschmann format is exploited for fluorescence-based sandwich immunoassay for quantitative detection of cTnI. The SPCE fluorescence chip utilizes the gold (2 nm)-silver (50 nm) bimetallic thin film, with which molecules of the dye Alexa 488 (conjugated with detection antibodies) make a near field coupling with the plasmonic film for SPCE. The experimental results find that the SPCE greatly improves the sensitivity via enhancing the fluorescence signal (up to 50-fold) while suppressing the photo-bleaching, permitting markedly enhanced signal-to-noise ratio. The limit of detection of 21.2 ag mL−1 (atto-gram mL−1) is obtained, the lowest ever reported to date amid those achieved by optical technologies such as luminescence and label-free optical sensing techniques. The features discovered such as ultrahigh sensitivity may prompt the presented technologies to be applied for early diagnosis of cTnI in blood, particularly for emergency medical centers overloaded with patients with acute myocardial infarction who would suffer from time-delayed diagnosis due to insufficient assay device sensitivity.

We present a label-free optical fiber based sensor device to detect copper ions (Cu2+) in water. A multimode optical fiber, with its polymer cladding removed along a 1-cm length, is used for the optical sensor head, where the injected Cu2+ in the liquid phase acts as a liquid cladding for the optical mode. The various Cu2+ concentrations modulate the numerical aperture (NA) of the liquid cladding waveguide part. The degree of NA mismatch between the liquid cladding and solid cladding guided parts gives rise to an optical power transmittance change, forming the sensing principle. The presented liquid cladding fiber sensor exhibits a minimum resolvable refractive index of 2.48 × 10−6. For Cu2+ detection, we functionalize the sensor head surface (fiber core) using chitosan conjugated ethylenediaminetetraacetic acid (EDTA) which captures Cu2+ effectively due to the enhanced chelating effects. We obtain a limit of detection of Cu2+ of 1.62 nM (104 ppt), which is significantly lower than the tolerable level in drinking water (~30 µM), and achieve a dynamic range of 1 mM. The simple structure of the sensor head and the sensing system ensures the potential capability of being miniaturized. This may allow for in-situ, highly-sensitive, heavy metal sensors in a compact format.

We demonstrate the enhancement of fluorescence emitted from dye molecules coupled with two surface plasmons, i.e., silver nanoparticles (AgNPs)-induced localized surface plasmons (LSP) and thin silver (Ag) film supported surface plasmons. Excitation light is illuminated to a SiO2 layer that contains both rhodamine 110 molecules and AgNPs. AgNPs enhances excitation rates of dye molecules in their close proximity due to LSP-induced enhancement of local electromagnetic fields at dye excitation wavelengths. Moreover, the SiO2 layer on one surface of which a 50 nm-thick Ag film is coated for metal cladding (air on the other surface), acts as a waveguide core at the dye emission wavelengths. The Ag film induces the surface plasmons which couple with the waveguide modes, resulting in a waveguide-modulated version of surface plasmon coupled emission (SPCE) for different SiO2 thicknesses in a reverse Kretschmann configuration. We find that varying the SiO2 thickness modulates the fluorescent signal of SPCE, its modulation behavior being in agreement with the theoretical simulation of thickness dependent properties of the coupled plasmon waveguide resonance. This enables optimization engineering of the waveguide structure for enhancement of fluorescent signals. The combination of LSP enhanced dye excitation and the waveguide-modulated version of SPCE may offer chances of enhancing fluorescent signals for a highly sensitive fluorescent assay of biomedical and chemical substances.

We experimentally demonstrate the spectral blue shift of surface plasmon resonance through the resonant coupling between quantum dots (QDs) and surface plasmons, surprisingly in contrast to the conventionally observed red shift of plasmon spectroscopy. Multimode optical fibers are used for extended resonant coupling of surface plasmons with excited states of QDs adsorbed to the plasmonic metal surface. The long-lived nature of excited QDs permits QD-induced negative change in the local refractive index near the plasmonic metal surface to cause such a blue shift. The analysis utilizes the physical causality-driven optical dispersion relation, the Kramers–Kronig (KK) relation, attempting to understand the abnormal behavior of the QDs-induced index dispersion extracted from blue shift measurement. Properties of QDs’ gain spectrally resonating with plasmons can account for such blue shift, though their absorbance properties never allow the negative index change for the blue shift observed according to the KK relation. We also discuss the limited applicability of the KK relation and possible QDs gain saturation for the experiment–theory disagreement. This work may contribute to the understanding of the photophysical properties critical for plasmonic applications, such as plasmonic local index engineering required in analyte labeling QDs coupled with plasmons for biomedical imaging or assay.