Determination of E. coli by a Graphene Oxide-Modified Quartz Crystal Microbalance

Toward the Real-Time and Rapid Quantification of Bacterial Cells Utilizing a Quartz Tuning Fork Sensor

The quantitative evaluation of bacterial populations is required in many studies, particularly in the field of microbiology. The current techniques can be time-consuming and require a large volume of samples and trained laboratory personnel. In this regard, on-site, easy-to-use, and direct detection techniques are desirable. In this study, a quartz tuning fork (QTF) was investigated for the real-time detection of E. coli in different media, as well as the ability to determine the bacterial state and correlate the QTF parameters to the bacterial concentration. QTFs that are commercially available can also be used as sensitive sensors of viscosity and density by determining the QTFs' damping and resonance frequency. As a result, the influence of viscous biofilm adhered to its surface should be detectable. First, the response of a QTF to different media without E. coli was investigated, and Luria-Bertani broth (LB) growth medium caused the largest change in frequency. Then, the QTF was tested against different concentrations of E. coli (i.e., 10²-10⁵ colony-forming units per milliliter (CFU/mL)). As the E. coli concentration increased, the frequency decreased from 32.836 to 32.242 kHz. Similarly, the quality factor decreased with the increasing E. coli concentration. With a coefficient (R) of 0.955, a linear correlation between the QTF parameters and bacterial concentration was established with a 26 CFU/mL detection limit. Furthermore, a considerable change in frequency was observed against live and dead cells in different media. These observations demonstrate the ability of QTFs to distinguish between different bacterial states. QTFs allow real-time, rapid, low-cost, and non-destructive microbial enumeration testing that requires only a small volume of liquid sample.

Chlorambucil-Loaded Graphene-Oxide-Based Nano-Vesicles for Cancer Therapy

In this study, the authors have designed biocompatible nano-vesicles using graphene oxide (GO) for the release of chlorambucil (CHL) drugs targeting cancerous cells. The GO sheets were first sulfonated and conjugated with folic acid (FA) molecules for controlled release and high loading efficiency of CHL. The chlorambucil (CHL) drug loading onto the functionalized GO surface was performed through π-π stacking and hydrophobic interactions with the aromatic planes of GO. The drug loading and "in vitro" release from the nano-vesicles at different pH were studied. The average particle size, absorption, and loading efficiency (%) of FA-conjugated GO sheets (CHL-GO) were observed to be 300 nm, 58%, and 77%, respectively. The drug release study at different pH (i.e., 7.4 and 5.5) showed a slight deceleration at pH 7.4 over pH 5.5. The amount of drug released was very small at pH 7.4 in the first hour which progressively increased to 24% after 8 h. The rate of drug release was faster at pH 5.5; initially, 16% to 27% in the first 3 h, and finally it reached 73% after 9 h. These observations indicate that the drug is released more rapidly at acidic pH with a larger amount of drug-loading ability. The rate of drug release from the CHL-loaded GO was 25% and 75% after 24 h. The biotoxicity study in terms of % cell viability of CHL-free and CHL-loaded GO against human cervical adenocarcinoma cell line was found to have lower cytotoxicity of CHL-loaded nano-vesicles (IC50 = 18 μM) as compared to CHL-free (IC50 = 8 μM). It is concluded that a high drug-loading efficiency and controlled release with excellent biotoxicity of CHL-GO offers an excellent application in the biomedical field.

Substrate-Free Untagged Detection of miR393a Using an Ultrasensitive Electrochemical Biosensor

Rapid and sensitive detection of numerous regulatory pathways in growth and development processes and defensive responses in plant-pathogen interactions caused by miRNA has been the current interest of agricultural scientists. Herein, an uncomplicated ultrasensitive electrochemical biosensor was fabricated to detect miR393a, as its detection is of vital importance for plant diseases. A streptavidin-coated screen-printed carbon electrode (SPCE) was fabricated and characterized by scanning electrochemical microscopy, scanning electron microscopy, surface plasmon resonance, and cyclic voltammetry. The two-dimensional (2D) structure and chemical functionality of the streptavidin-coated SPCE render it a superior platform for loading a modified probe via a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide-N-hydroxysuccinimide linker. This biorecognition platform is capable of efficiently using its excellent conductivity, greater surface area, and effective electrochemical execution due to its synergistic effect between streptavidin and carbon electrodes. The biosensor showed a good linear response (R ² = 0.96) to miR393a concentrations ranging from 100 nM to 100 fM. This streptavidin-based biosensor is highly sensitive to the minimum concentration of miR393a, lowest detection limit, and ultrasensitivity under optimized conditions, i.e., 100 fM, 0.33 fM, and 33.72 μA fM^-1 cm^-2, respectively. In addition, remarkable recoveries could be obtained to confirm the feasibility of this assay in plant disease samples. The fabricated technology could offer a selective, adaptable, and farmer-friendly strategy for the timely detection of miRNA of plant samples.

Low concentration E. coli O157:H7 bacteria sensing using microfluidic MEMS biosensor

This paper reports the design, fabrication, and testing of a microfluidic MEMS biosensor for rapid sensing of low concentration Escherichia coli O157:H7. It consists of a specially designed focusing and sensing region, which enables the biosensor to detect low concentration of bacterial cells. The focusing region consists of a ramped vertical electrode pair made of electroplated gold along with tilted thin film finger pairs (45°) embedded inside a microchannel. The focusing region generates positive dielectrophoresis force, which moves the cells towards the edges of the tilted thin film electrode fingers, located at the center of the microchannel. The fluidic drag force then carries the focused cells to the sensing region, where three interdigitated electrode arrays (IDEAs) with 30, 20, and 10 pairs, respectively, are embedded inside the microchannel. This technique resulted in highly concentrated samples in the sensing region. The sensing IDEAs are functionalized with the anti-E. coli antibody for specific sensing of E. coli 0157:H7. As E. coli binds to the antibody, it results in an impedance change, which is measured across a wide frequency range of 100 Hz-10 MHz. The biosensor was fabricated on a glass substrate using the SU8 epoxy resist to form the microchannel, gold electroplating to form the vertical focusing electrode pair, a thin gold film to form the sensing electrode, the finger electrodes, traces and bonding pads, and polydimethylsiloxane to seal the device. The microfluidic impedance biosensor was tested with various low concentration bacterial samples and was able to detect bacterial concentration, as low as 39 CFU/ml with a total sensing time of 2 h.

Graphene Oxide-Polycarbonate Track-Etched Nanosieve Platform for Sensitive Detection of Human Immunodeficiency Virus Envelope Glycoprotein

Solid-state nanopores within graphene-based materials are on the brink of fundamentally changing the sensing of desired bioanalytes through ion trafficking across nanoporous membranes. Here, we report on a two-electrode electrochemical biosensor comprised of a graphene oxide-polycarbonate track-etched nanosieve platform for the rapid and sensitive detection of the Human Immunodeficiency Virus Type 1 (HIV-1) envelope glycoprotein ectodomain (gp140_MS). We have covalently linked an engineered high-affinity one-domain soluble CD4 fused to a human domain targeting HIV-1 coreceptor binding site and ferrocene (Fc) (2Dm2m) to the nanosieve platform. An exponential decrease in the ionic current resulted from a partial blockade of the nanosieve due to the specific interactions of gp140_MS with the 2Dm2m protein, which was immobilized on the nanosieve platform by biolinkage as a function of applied voltages of 0.1-2.0 V. There was no change in current when a nonspecific antigen bovine serum albumin was tested under identical conditions. This platform had high sensitivity, and when the receptor-binding phenomenon was tested to identify the minimum concentration of target analyte, the lowest detection limit was as short as 8.3 fM and with sensitivity and response times of 0.87 mA mM^-1 cm^-1 and 12 s, respectively. In addition to this remarkable sensitivity, our nanobiorecognition platform has the advantage of superior stability due to the few layered graphene oxide laminates. It also exhibits exceptional biomolecule binding and higher reusability, sustainability, and ease of fabrication in a soft mechanism. Real samples of HIV positive and negative patients were successfully tested to confirm the virus by the developed platform. To the best of our knowledge, this is the first time prosperous pervious remembrance surface has been employed in a nanobiosensing application. In light of the recent great trend of using graphene-based nanopore surfaces created by sophisticated ion-beam methods in sensing and sequencing, this hybrid-surface nanolayer fabricated by the simple vacuum filtration of a few layered graphene oxide laminates may serve as a good alternative in terms of ease of fabrication without expensive instrumental prerequisites.