An ultra-sensitive SARS-CoV-2 antigen optical biosensor based on angiotensin converting enzyme 2 (ACE-2) functionalized magnetic-fluorescent silica nanoparticles.

This work reports on the design and synthesis of an angiotensin-converting enzyme 2 (ACE-2) functionalized magnetic fluorescent silica nanoparticles (Fe-FSNP) as a biosensing platform to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen. Iron oxide (Fe3O4) nanoparticles were synthesized via ultrasonic-assisted coprecipitation and then coated with fluorescent silica nanoparticles (FSNP) through thesol-gelmethod forming the Fe-FSNP samples. Silica obtained from local geothermal powerplant was used in this work and Rhodamine B was chosen as the incorporated fluorescent dye, hence this reports for the first time ACE-2 was immobilized on the natural silica surface. The Fe-FSNP nanoparticle consists of a 18-25 nm magnetic core and a silica shell with a thickness of 30 nm as confirmed from the transmission electron microscopy image. Successful surface functionalization of the Fe-FSNP with ACE-2 as bioreceptor was conducted through hydrosylilation reaction and confirmed through the Fourier transform infrared spectroscopy. The detection of SARS-Cov-2 antigen by Fe-FSNP/ACE2 was measured through the change in its maximum fluorescence intensity at 588 nm where fluorescence- quenching had occurred. The biosensing platform showed a rapid response at 30 min with a linear range of 10-6to 10-2µg ml-1. The magnetic-fluorescent properties of the nanoparticle enables an ultra-sensitive detection of SARS-Cov-2 antigen with the limit of detection as low as 2 fg ml-1.

Optimized antibody immobilization on natural silica-based nanostructures for the selective detection of E. coli.

This study reports for the first time the surface modification of fluorescent nanoparticles derived from geothermal silica precipitate with Escherichia coli (E. coli) antibody. The immobilization of biomolecules on the inorganic surface has been carried out using two different pathways, namely the silanization and hydrosilylation reactions. The former applied (3-aminopropyl)triethoxysilane (APTES) as the crosslinker, while the latter used N-hydroxysuccinimide coupled with N-ethyl-N'-(3-dimethyl aminopropyl) carbodiimide hydrochloride (EDC/NHS). Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FESEM-EDX), and fluorescence spectroscopy were used to confirm the chemical, physical, and optical properties of the surface-modified fluorescent silica nanoparticles (FSNPs). Based on the results of the FTIR, fluorescence spectroscopy and stability tests, the modified FSNPs with EDC/NHS with a ratio of 4 : 1 were proven to provide the optimum results for further conjugation with antibodies, affording the FSNP-Ab2 sample. The FSNP-Ab2 sample was further tested as a nanoplatform for the fluorescence-quenching detection of E. coli, which provided a linear range of 102 to 107 CFU mL-1 for E. coli with a limit of detection (LoD) of 1.6 × 102 CFU mL-1. The selectivity of the biosensor was observed to be excellent for E. coli compared to that for P. aeruginosa and S. typhimurium, with reductions in the maximum fluorescence intensity at 588 nm of 89.22%, 26.23%, and 54.06%, respectively. The inorganic nanostructure-biomolecule conjugation with optimized coupling agents showed promising analytical performance as a selective nanoplatform for detecting E. coli bacteria.

Rapid Fluorescence Quenching Detection of Escherichia coli Using Natural Silica-Based Nanoparticles.

The development of fluorescent silica nanoparticles (SNP-RB) from natural amorphous silica and its performance as an Escherichia coli (E. coli) biosensor is described in this paper. SNP-RB was derived from silica recovered from geothermal installation precipitation and modified with the dye, Rhodamine B. The Fourier Infrared (FTIR) confirms the incorporation of Rhodamine B in the silica matrix. Transmission Electron Microscopy (TEM) micrographs show that the SNP-RB had an irregular structure with a particle diameter of about 20-30 nm. The maximum fluorescence spectrum of SNP-RB was recorded at 580 nm, which was further applied to observe the detection performance of the fluorescent nanoparticles towards E. coli. The sensing principle was based on the fluorescence-quenching mechanism of SNP-RB and this provided a wide linear E. coli concentration range of 10-105 CFU/mL with a limit detection of 8 CFU/mL. A rapid response time was observed after only 15 min of incubation of SNP-RB with E. coli. The selectivity of the biosensor was demonstrated and showed that the SNP-RB only gave quenching response only to live E. coli bacteria. The use of SNP-RB as a sensing platform reduced the response time significantly compared to conventional 3-day bacterial assays, as well having excellent analytical performance in terms of sensitivity and selectivity.

Singlet Oxygen Detection on a Nanostructured Porous Silicon Thin Film via Photonic Luminescence Enhancements.

Because reactive oxygen species are involved in a range of pathologies, developing analytical tools for this group of molecules opens new vistas for biomedical diagnostics. Herein, we fabricate a porous silicon microcavity (pSiMC) functionalized with luminescent singlet oxygen (1O2) probe EuA ((Eu(III)-2,2',2″-(10-(2-((4-(2-((4-(2-((anthracen-9-ylmethyl)amino)ethyl)-1H-1,2,3-triazol-1-yl)amino)-2-oxoethyl)-2-oxo-1,2-dihydroquinolin-7-yl)amino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid) as proof of concept of an optical sensor for reactive oxygen species. We characterize each surface modification step of the pSiMC by means of FTIR and X-ray photoelectron spectroscopy as well as by determining the optical shifts of the resonance wavelength of the pSiMC. The luminescence signal upon detection of 1O2 on the EuA-modified pSiMC is enhanced ∼2-fold compared to that of a single layer and a detuned microcavity. The sensing performance of the EuA probe is improved significantly on the pSiMC compared to that in aqueous solution, giving a limit of 1O2 detection of 3.7 × 10-8 M.

Recent Advances on Luminescent Enhancement-Based Porous Silicon Biosensors.

Luminescence-based detection paradigms have key advantages over other optical platforms such as absorbance, reflectance or interferometric based detection. However, autofluorescence, low quantum yield and lack of photostability of the fluorophore or emitting molecule are still performance-limiting factors. Recent research has shown the need for enhanced luminescence-based detection to overcome these drawbacks while at the same time improving the sensitivity, selectivity and reducing the detection limits of optical sensors and biosensors. Nanostructures have been reported to significantly improve the spectral properties of the emitting molecules. These structures offer unique electrical, optic and magnetic properties which may be used to tailor the surrounding electrical field of the emitter. Here, the main principles behind luminescence and luminescence enhancement-based detections are reviewed, with an emphasis on europium complexes as the emitting molecule. An overview of the optical porous silicon microcavity (pSiMC) as a biosensing platform and recent proof-of-concept examples on enhanced luminescence-based detection using pSiMCs are provided and discussed.

Development of L-lactate dehydrogenase biosensor based on porous silicon resonant microcavities as fluorescence enhancers.

The up-regulation of L-lactate dehydrogenase (LDH), an intracellular enzyme present in most of all body tissues, is indicative of several pathological conditions and cellular death. Herein, we demonstrate LDH detection using porous silicon (pSi) microcavities as a luminescence-enhancing optical biosensing platform. Non-fluorescent resazurin was covalently attached onto the pSi surface via thermal hydrocarbonisation, thermal hydrosylilation and acylation. Each surface modification step was confirmed by means of FTIR and the optical shifts of the resonance wavelength of the microcavity. Thermal hydrocarbonisation also afforded excellent surface stability, ensuring that the resazurin was not reduced on the pSi surface. Using a pSi microcavity biosensor, the fluorescence signal upon detection of LDH was amplified by 10 and 5-fold compared to that of a single layer and a detuned microcavity, respectively, giving a limit of detection of 0.08 U/ml. The biosensor showed a linear response between 0.16 and 6.5 U/ml, covering the concentration range of LDH in normal as well as damaged tissues. The biosensor was selective for LDH and did not produce a signal upon incubation with another NAD-dependant enzyme L-glutamic dehydrogenase. The use of the pSi microcavity as a sensing platform reduced reagent usage by 30% and analysis time threefold compared to the standard LDH assay in solution.

Lanthanide luminescence enhancements in porous silicon resonant microcavities.

In this paper, the covalent immobilization and luminescence enhancement of a europium (Eu(III)) complex in a porous silicon (pSi) layer with a microcavity (pSiMC) structure are demonstrated. The alkyne-pendant arm of the Eu(III) complex was covalently immobilized on the azide-modified surface via ligand-assisted "click" chemistry. The design parameters of the microcavity were optimized to obtain an efficient luminescence-enhancing device. Luminescence enhancements by a factor of 9.5 and 3.0 were observed for Eu(III) complex bound inside the pSiMC as compared to a single layer and Bragg reflector of identical thickness, respectively, confirming the increased interaction between the immobilized molecules and the electric field in the spacer of the microcavity. When comparing pSiMCs with different resonance wavelength position, luminescence was enhanced when the resonance wavelength overlapped with the maximum emission wavelength of the Eu(III) complex at 614 nm, allowing for effective coupling between the confined light and the emitting molecules. The pSiMC also improved the spectral color purity of the Eu(III) complex luminescence. The ability of a pSiMC to act as an efficient Eu(III) luminescence enhancer, combined with the resulting sharp linelike emission, can be exploited for the development of ultrasensitive optical biosensors.

Biomolecule detection in porous silicon based microcavities via europium luminescence enhancement.

In this paper, we demonstrate the detection of europium-complex-labeled streptavidin in a porous silicon microcavity (pSiMC) via luminescence enhancement. The pSiMC platform was modified for optimized luminescence enhancement which encompassed changing the pore size of the microcavity to ensure molecular infiltration and adjusting the optical quality of the microcavity. Characterization of the optimized surface was performed by infrared spectroscopy, interferometric reflectance spectroscopy and luminescence measurements. Luminescence enhancement of the bound Eu(iii) complex by a factor of 3 was observed on the optimized pSiMC as compared to that on a single pSi layer. The ability of a pSiMC to act as a luminescence enhancing sensor was confirmed using streptavidin as a model analyte on a biotin-modified pSiMC. The sensor was able to detect Eu(iii) complex labeled streptavidin with a concentration as low as 150 nM. Furthermore, streptavidin was selectively detected when spiked in human wound fluid. The concept of detecting Eu(iii) labeled bioconjugates on pSiMC may be incorporated into the design of highly sensitive and specific point-of-care biosensors.