A review on electronic bio-sensing approaches based on non-antibody recognition elements

In this review, recent advances in the development of electronic detection methodologies based on non-antibody recognition elements such as functional liposomes, aptamers and synthetic peptides are discussed. Particularly, we highlight the progress of field effect transistor (FET) sensing platforms where possible as the number of publications on FET-based platforms has increased rapidly. Biosensors involving antibody-antigen interactions have been widely applied in diagnostics and healthcare in virtue of their superior selectivity and sensitivity, which can be attributed to their high binding affinity and extraordinary specificity, respectively. However, antibodies typically suffer from fragile and complicated functional structures, large molecular size and sophisticated preparation approaches (resource-intensive and time-consuming), resulting in limitations such as short shelf-life, insufficient stability and poor reproducibility. Recently, bio-sensing approaches based on synthetic elements have been intensively explored. In contrast to existing reports, this review provides a comprehensive overview of recent advances in the development of biosensors utilizing synthetic recognition elements and a detailed comparison of their assay performances. Therefore, this review would serve as a good summary of the efforts for the development of electronic bio-sensing approaches involving synthetic recognition elements

Detection of matrilysin (MMP-7) activity using polypeptide functionalized reduced graphene oxide field-effect transistor sensor.

A novel approach for rapid and sensitive detection of matrilysin (MMP-7, a biomarker involved in the degradation of vari-ous macromolecules) based on polypeptide (JR2EC) functionalized reduced graphene oxide (rGO) field effect transistor (FET) is reported. MMP-7 specifically digests negatively charged JR2EC immobilized on rGO, thereby modulating the con-ductance of rGO-FET. The proposed assay enabled detection of MMP-7 at clinically relevant concentrations with a limit of detection (LOD) of 10 ng/mL (400 pM), attributed to the significant reduction of the net charge of JR2EC upon digestion by MMP-7. Quantitative detection of MMP-7 in human plasma was further demonstrated with a LOD of 40 ng/mL, illustrating the potential for the proposed methodology for tumor detection and carcinoma diagnostic (e.g. lung cancer and salivary gland cancer). Additionally, excellent specificity of the proposed assay was demonstrated using matrix metallopeptidase 1 (MMP-1), a protease of the same family. With appropriate selection and modification of polypeptides, the proposed assay could be extended for detections of other enzymes with polypeptide digestion capability

Curvature of the Localized Surface Plasmon Resonance Peak

Localized surface plasmon resonance (LSPR) occurring in noble metal nanoparticles (e.g., Au) is a widely used phenomenon to report molecular interactions. Traditional LSPR sensors typically monitor shifts in the peak position or extinction in response to local refractive index changes in the close vicinity of the nanoparticle surface. The ability to resolve minute shifts/extinction changes is to a large extent limited by instrumental noise. A new strategy to evaluate LSPR responses utilizing changes in the shape of the extinction spectrum (the curvature) is proposed. The response of curvature to refractive index changes is investigated theoretically using Mie theory and an analytical expression relating the curvature to the refractive index is presented. The experimentally derived curvatures for 13 nm spherical gold nanoparticles (AuNPs) exposed to solvents with different bulk refractive indices confirm the theoretical predictions. Moreover, both the calculated and experimental findings suggest that the curvature is approximately a linear function of refractive index in regimes relevant to bio and chemical sensing. We demonstrate that curvature is superior over peak shift and extinction both in terms of signal-to-noise (S/N) ratio and reliability of LSPR sensors. With a curvature, one could readily monitor submonolayer adsorption of a low molecular weight thiol molecule (<i>M</i>_w = 458.6) onto 13 nm AuNPs. It is also worthwhile mentioning that curvature is virtually insensitive to instrumental instabilities and artifacts occurring during measurement. Instabilities such as baseline tilt and shift, shift in peak position as well as sharp spikes/steps in the extinction spectra do not induce artifacts in the sensorgrams of curvature

Luminescent Device for the Detection of Oxidative Stress Biomarkers in Artificial Urine

A luminescent paper-based device for the visual detection of oxidative stress biomarkers is reported. The device consists of a polyvinylidene fluoride membrane impregnated with poly­(3-alkoxy-4-methylthiophene) (PT) for colorimetric detection. 8-hydroxy-2′-deoxyguanosine (8-OHdG), a biomarker associated with oxidative stress, is used as a model system for validating the proposed methodology. The detection strategy is based on monitoring the changes in optical properties of PT associated with its conformational changes upon interaction with an aptamer in the presence and in the absence of 8-OHdG. Fluorometric and colorimetric monitoring revealed linear responses for 8-OHdG concentrations between 50 pM and 500 nM (∼14 pg/mL to 140 ng/mL), with limits of detection of ∼300 pM and  ∼350 pM, respectively for (<i>n</i> = 3). Colorimetric responses in artificial urine ascertained rapid, sensitive, and selective detection of 8-OHdG at clinically relevant (pM to nM) concentration levels. Furthermore, the proposed methodology enables point-of-care diagnostics for oxidative stress without requiring sophisticated instrumentation

Highly Sensitive, Label-Free Detection of 2,4-Dichlorophenoxyacetic Acid Using an Optofluidic Chip

A highly sensitive approach for rapid and label-free detection of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) using an optofluidic chip is demonstrated. The optofluidic chip is prepared by covalent immobilization of 2,4-D-bovine serum albumin (2,4-D-BSA) conjugate to an integrated microring resonator. Subsequent detection of 2,4-D carried out in a competitive immunoreaction format enables selective detection of 2,4-D in different types of water samples, including bottled, tap, and lake water, at a limit of detection (LOD) of 4.5 pg/mL and in a quantitative range of 15–10⁵ pg/mL. The microring resonator-based optofluidic chip is reusable with ultrahigh sensitivity that offers real-time and on-site detection of low-molecular-weight targets for potential applications in food safety and environmental monitoring

Inflection Point of the Localized Surface Plasmon Resonance Peak: A General Method to Improve the Sensitivity

The shift of the localized surface plasmon resonance (LSPR) spectrum is widely used in bio- and chemical sensing. Traditionally, the shift is monitored at the peak maximum of the extinction spectrum. We demonstrate that the inflection point at the long wavelength side of the peak maximum shows better refractive index sensitivity than the peak maximum. A consistent improvement in bulk refractive index sensitivity of 18–55% is observed for six different nanoparticles such as spherical particles of different sizes, nanostar and nanorods with different aspect ratios. Local refractive index changes induced by molecular adsorption confirm the superior performance of the method. We contribute this improvement in sensitivity to the change in shape of the LSPR peak in response to an increase of the local refractive index. We further illustrate the advantage of using the inflection point method for analyzing DNA adsorption on U-shaped metamaterials, and for using 17 nm spherical gold nanoparticles for detection of matrix metalloprotease 7 (MMP-7), a biomarker that is heavily up-regulated during certain cancers. With the inflection point, the limit of detection (LOD) for MMP-7 is improved to 0.094 μg/mL from 0.22 μg/mL. This improvement may facilitate early diagnosis of salivary and colorectal cancers. We also envision that this generic method can be employed to track minute optical responses in other analytical areas

Peptide-Assembled Graphene Oxide as a Fluorescent Turn-On Sensor for Lipopolysaccharide (Endotoxin) Detection

Lipopolysaccharide (LPS) is a toxic inflammatory stimulator released from the outer cell membrane of Gram-negative bacteria, known to be directly related to, for example, septic shock, that causes millions of casualties annually. This number could potentially be lowered significantly if specific, sensitive, and more simply applicable LPS biosensors existed. In this work, we present a facile, sensitive and selective LPS sensor, developed by assembling tetramethylrhodamine-labeled LPS-binding peptides on graphene oxide (GO). The fluorescence of the dye-labeled peptide is quenched upon interaction with GO. Specific binding to LPS triggers the release of the peptide-LPS complex from GO, resulting in fluorescence recovery. This fluorescent turn-on sensor offers an estimated limit of detection of 130 pM, which is the lowest ever reported among all synthetic LPS sensors to date. Importantly, this sensor is applicable for detection of LPS in commonly used clinical injectable fluids, and it enables selective detection of LPS from different bacterial strains as well as LPS on the membrane of living E. coli

Assembly of Graphene Oxide and Au_0.7Ag_0.3 Alloy Nanoparticles on SiO₂: A New Raman Substrate with Ultrahigh Signal-to-Background Ratio

Resonance Raman spectroscopy (RRS) often suffers from the large fluorescence background which obscures the much weaker Raman scattering. To address this fundamental problem, a novel Raman substrate has been fabricated by adsorption of Au_0.7Ag_0.3 alloy nanoparticles (NPs) on a graphene oxide (GO) coated SiO₂ surface, which offers both excellent Raman enhancement and fluorescence quenching. Our experimental data reveal that a Raman to fluorescence background intensity ratio of 1.6 can be obtained for a highly fluorescent dye like Alexa fluor 488. Moreover, we demonstrate that the Raman enhancement mainly originates from the Au_0.7Ag_0.3 alloy NPs and that the fluorescence quenching mainly arises from the underlying functionalized GO (FGO) substrate

Screening Criteria for Qualified Antibiotic Targets in Unmodified Gold Nanoparticles-Based Aptasensing

In designing unmodified gold nanoparticles-based aptasensing (uGA) assays for antibiotics, we find that some antibiotics can adsorb directly on gold nanoparticles (GNP) regardless of the presence of aptamers, which have been long overlooked in the past. Some adsorptions, however, would strongly disturb the charge distribution on the GNP surface, break up the static colloidal profile, and thus generate false positive colorimetric signals. To identify antibiotics qualified for uGA assays, we established two rational screening criteria for antibiotic targets relying on their oil–water partition coefficients (log <i>P</i> values) and net physiological charges: log <i>P</i> > 0 and charge ≤0. A good agreement of the GNP color change was obtained between the two criteria-based predictions and the actual tests using six representative antibiotics. The proposed criteria help to shed light on GNP–target interactions, which is significant for developing novel GNP-based colorimetric assays with high reliability

Observation of Stripe Superstructure in the β‑Two-Phase Coexistence Region of Cholesterol–Phospholipid Mixtures in Supported Membranes

Visualization of phase coexistence in the β region of cholesterol–phospholipid mixtures consisting of high cholesterol concentrations has proved elusive in lipid bilayers. Here, using the solvent-assisted lipid bilayer approach to prepare supported membranes with high cholesterol fractions close to the cholesterol solubility limit, we report the observation of coexisting liquid phases using fluorescence microscopy. At ∼63 mol % cholesterol, supported membranes consisting of mixtures of DOPC and cholesterol exhibit large-area striping reminiscent of the stripe superstructures that characterize the proximity of the second critical point in the miscibility phase diagram. The properties of the two phases are consistent with condensed complex-rich and cholesterol-rich liquids. Both phases exhibit long-range lateral mobility, and diffusion through a given phase is favored over hopping across the phase boundary, producing an “archipelago effect” and a complex percolation path