Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

No abstract

A novel water-compatible molecularly imprinted polymer (MIP), prepared with enrofloxacin (ENR) as the template, has been optimised for the selective extraction of fluoroquinolone antibiotics in aqueous media. The results of a morphological characterisation and selectivity tests of the polymer material for ENR and related derivatives are reported. High affinity for the piperazine-based fluoroquinolones marbofloxacin, ciprofloxacin, norfloxacin and ofloxacin was observed, whereas no retention was found for nonrelated antibiotics. Various parameters affecting the extraction efficiency of the polymer have been optimised to achieve selective extraction of the antibiotics from real samples and to reduce nonspecific interactions. These findings resulted in a MISPE/HPLC-FLD method allowing direct extraction of the analytes from aqueous samples with a selective wash using just 50% (v/v) organic solvent. The method showed excellent recoveries and precision when buffered urine samples fortified at five concentration levels (25-250 ng mL(-1) each) of marbofloxacin, ciprofloxacin, norfloxacin, enrofloxacin and sarafloxacin were tested (53-88%, RSD 1-10%, n = 3). Moreover, the biological matrix of the aqueous samples did not influence the preconcentration efficiency of the fluoroquinolones on the MIP cartridges; no significant differences were observed between the recovery rates of the antibiotics in buffer and urine samples. The detection limits of the whole process range between 1.9 and 34 ng mL(-1) when 5-mL urine samples are processed. The developed method has been successfully applied to preconcentration of norfloxacin in urine samples of a medicated patient, demonstrating the ability of the novel MIP for selective extraction of fluoroquinolones in urine samples.

We describe the preparation of multibranched gold−silica−molecularly imprinted polymer (bAu@mSiO 2 @ MIP) core−shell nanoparticles, with their specific ability to recognize enrofloxacin (ENRO), and their application as labelfree nanosensors for the specific detection of the antimicrobial by surface-enhanced Raman scattering. The use of these nanocomposites results in a large enhancement of the Raman scattering of ENRO upon binding of an antibiotic to the selective recognition sites in the MIP. These are in the proximity of the gold core branches that act as intrinsic hot spots providing highly localized and strongly enhanced electromagnetic fields caused by plasmon resonance. The effect of the multibranched morphology of the gold cores (bAu) on the optical spectroscopic response of the bAu@mSiO 2 @MIP nanosensors is investigated with the aim of improving ENRO detection. The optimized nanostructures allowed us to achieve a detection limit of 1.5 nM for ENRO, which is 2 orders of magnitude lower than those for previously reported Au@MIP nanosensors, additionally providing negligible cross-reactivity toward other potential interfering species.

The present work describes the development of a sensitive and highly selective innovative method for the simultaneous detection of six fluoroquinolone (FQ) antimicrobials (enrofloxacin, ciprofloxacin, norfloxacin, levofloxacin, danofloxacin, and sarafloxacin) in water samples. This detection is based on online solid phase extraction, coupled to liquid chromatography (LC), using for the first time tailor-made molecularly imprinted microspherical polymer particles prepared via precipitation polymerization. Various parameters affecting the extraction efficiency of the polymer have been optimized to reduce nonspecific interactions and to achieve selective uptake of the antibiotics from real samples. The method shows good recoveries ranging between 62% and 102% (V = 25 mL) for the different FQs tested and excellent interday and intraday precision with relative standard deviation (RSD) values between 2-5% and 2-6%, respectively. The detection limits were between 1-11 ng L(-1) (drinking water) and 1-12 ng L(-1) (fish farm water) when 25 mL samples were processed. The polymer showed selectivity for FQs containing a piperazine moiety whereas no retention was found for other antibiotics or nonrelated compounds. The method has been applied to the analysis of trace amounts of the FQs tested in drinking and fish farm water samples with excellent recoveries (>91%) and good precision (RSDs <5%).

The application of optical biosensors, specifically those that use optical fibers and planar waveguides, has escalated throughout the years in many fields, including environmental analysis, food safety and clinical diagnosis. Fluorescence is, without doubt, the most popular transducer signal used in these devices because of its higher selectivity and sensitivity, but most of all due to its wide versatility. This paper focuses on the working principles and configurations of fluorescence-based fiber optic and planar waveguide biosensors and will review biological recognition elements, sensing schemes, as well as some major and recent applications, published in the last ten years. The main goal is to provide the reader a general overview of a field that requires the joint collaboration of researchers of many different areas, including chemistry, physics, biology, engineering, and material science.

Bacteriophage-based bioassays are a promising alternative to traditional antibody-based immunoassays. Bacteriophages, shortened to phages, can be easily conjugated or genetically engineered. Phages are robust, ubiquitous in nature, and harmless to humans. Notably, phages do not usually require inoculation and killing of animals; and thus, the production of phages is simple and economical. In recent years, phage-based biosensors have been developed featuring excellent robustness, sensitivity, and selectivity in combination with the ease of integration into transduction devices. This review provides a critical overview of phage-based bioassays and biosensors developed in the last few years using different interrogation methods such as colorimetric, enzymatic, fluorescence, surface plasmon resonance, quartz crystal microbalance, magnetoelastic, Raman, or electrochemical techniques.

One-half of the 2018 Nobel Prize in Chemistry was awarded jointly to George P. Smith and Sir Gregory P. Winter Bfor the phage display of peptides and antibodies^. This feature article summarizes significant achievements leading to the development of phage display of peptides and antibodies, where a bacteriophage is genetically modified to display peptides and proteins, with the primary aim of producing new biopharmaceuticals. These significant achievements are proven to be useful for the development of phage-based bioassays and biosensors.