Flexible Ag–C₆₀nano-biosensors based on surface plasmon coupled emission for clinical and forensic applications

Abstract

The relatively low sensitivity of fluorescence detection schemes, which are mainly limited by the isotropic nature of fluorophore emission, can be overcome by utilizing surface plasmon coupled emission (SPCE). In this study, we demonstrate directional emission from fluorophores on flexible Ag-C60 SPCE sensor platforms for point-of-care sensing, in healthcare and forensic sensing scenarios, with at least 10 times higher sensitivity than traditional fluorescence sensing schemes. Adopting the highly sensitive Ag-C60 SPCE platform based on glass and novel low-cost flexible substrates, we report the unambiguous detection of acid-fast Mycobacterium tuberculosis (Mtb) bacteria at densities as low as 20 Mtb mm(-2); from non-acid-fast bacteria (e.g., E. coli and S. aureus), and the specific on-site detection of acid-fast sperm cells in human semen samples. In combination with the directional emission and high-sensitivity of SPCE platforms, we also demonstrate the utility of smartphones that can replace expensive and cumbersome detectors to enable rapid hand-held detection of analytes in resource-limited settings; a much needed critical advance to biosensors, for developing countries.

Reproducible Enhancement of Fluorescence by Bimetal Mediated Surface Plasmon Coupled Emission for Highly Sensitive Quantitative Diagnosis of Double‐Stranded DNA

“…The dye molecules in a range of 10–100 nm of continuous surface of a thin metal film give rise to fluorescence enhancement by way of the three consecutive processes as follows: 1) A prism‐based ATR can produce large local fields of SPR evanescent waves that in turn excite the dye molecules within an evanescent field decaying length at λ ex , resulting in enhanced excitation rate . On the other hand, in a case that excites dyes by direct illumination from above the metal suface, no such plasmonic enhancement of excitation but normal excitaiton occurs at λ ex ; 2) An excited state of a dye molecule couples with surface plasmons at nearly the same as λ em , thus reducing its life‐time significantly; 3) The plasmonic backcoupling into the propagating photons through a high‐index transparent medium (such as a prism or a slide glass) leads to an intense Stoke‐shifted radiation cone, i.e., surface plasmon coupled emission (SPCE) . All these processes involved can eventually produce the enhancement of both fluorescent luminance and its collection efficiency, additionally benefitting from photobleaching reduction and autofluorescence suppression.…”

Chemiplasmonics for high-throughput biosensors

“…This enhancement, unlike the physical mechanism in case of Ag-RhB, arises due to the chemical interactions between C 60 and RhB molecules. We surmise that the π-orbitals in C 60 and RhB can result in π-π stacking 8 and thereby influence the ME in Equation 1 by restricting those rotational/vibrational modes of RhB molecules that would otherwise participate in the nonradiative decay of excited electrons. We obtained adsorption isotherms of RhB on Ag/C 60 substrates to confirm the presence of π-π interactions ( Figure 3C).…”

“…Although many previous studies explained the emission enhancements through the MEF model, changes in the spectral features, such as in Figure 1A, were partly ignored. 8,9,12,13,22,23 While the emission spectrum of RhB on 25 nm Ag required a two-peak fit, at least three to four peaks were necessary to fit the emission spectrum of RhB on Ag/ C 60 layers. This important fact is also reflected in Figure 2A-C, which show fluorescence microscope images of RhB coated on bare glass, 25 nm Ag, and 25 nm Ag/20 nm C 60 .…”

“…6 In the last two decades, surface plasmons from metallic nanoparticles (NPs) have been widely used to improve the detection limits for many biomarkers through fluorescence enhancements. [7][8][9][10][11][12][13][14][15] Metallic NPs are known to alter fluorescence emission of dyes by influencing the radiative of dye molecules within their vicinity. 16 The radiative decay rate (γ) of a dye molecule, from an excited state k  of energy E k to a lower state n  of energy E n , through the emission of a photon of energy ω , may be understood in terms of the Fermi's golden rule given by…”

“…18,19 Previously, we and others have demonstrated surface-plasmon-coupled emission sensors that enhance the fluorescence of dyes by increasing PMD in the presence of Ag/Au NPs and thin films. [7][8][9][10][11][12][13][14][15] Here, we posit that it is also plausible to enhance fluorescence through chemical interactions between the fluorescent molecule and the photonic environment that change the matrix element (ME)   n p E k   ⋅ in Equation 1 in addition to increasing PMD.…”