Biosensors are analytical devices that are able to convert a biological response into
an electrical signal. The “golden” biosensor must be highly specific, independent
of physical parameters (e.g., pH, temperature, etc.), and should be reusable. The
research within the biosensing field requires a multidisciplinary approach that involves
different branches of science such as chemistry, biology, and engineering. Biosensors
can be categorized based on the biorecognition mechanism: with the biocatalytic group
comprising enzymes, the bio-affinity group including antibodies and nucleic acids,
and the microbe-based group containing microorganisms. The present Special Issue aimed
at summarizing the most recent findings and future challenges regarding biosensors.
In the last six decades, several biosensors have been reported as end-user and time-saving
analytical methods for the detection of multiple analytes (e.g., food, clinical, and
environmental analytes). In 1962, Professor Leland C. Clark published the first example
of an enzyme electrochemical biosensor by entrapping glucose oxidase in a dialysis
membrane over a Clark-type oxygen electrode [1]. Moreover, Guilbault and Montalvo
reported on glass electrodes coupled with urease to measure urea concentration by
means of potentiometry [2]. Besides these first examples, electrochemical transducers
have been combined with enzymes, antibodies, and DNA as biochemical recognition components.
Nowadays, they represent the largest category of biosensors for food, clinical, and
environmental sensing.
The increasing number of scientific publications focusing on biosensors indicates
growing interest in the broader scientific community (Figure 1). The present collection
of the papers is devoted to all aspects of biosensing in a very broad definition,
including, but not limited to, biomolecular composition used in biosensors (e.g.,
biocatalytic enzymes, DNAzymes, abiotic nanospecies with biocatalytic features, bioreceptors,
DNA/RNA, aptasensors, etc.), physical signal transduction mechanisms (e.g., electrochemical,
optical, magnetic, etc.), engineering of different biosensing platforms, operation
of biosensors in vitro and in vivo (implantable or wearable devices), self-powered
biosensors, etc. The biosensors can be represented with analogue devices measuring
concentrations of analytes and binary devices operating in the YES/NO format, possibly
with logical processing of input signals.
In this collection, we combined twenty outstanding contributions focusing on different
aspects of the biosensing field, mostly highlighting recent advances and future challenges
of DNA detection, immunosensing, in vivo electrochemical biosensors, redox enzyme-modified
electrode surfaces, photoelectrochemical processes, field-effect transistor-based
biosensors, etc., which can be considered as biosensing sub-topics, as reported in
Figure 2. A brief summary of each accepted contribution is provided below to encourage
the readers to go through them and “visualize” the state of the art within the field
of biosensing.
Among the big question marks in biosensing development, Vadgama certainly addressed
one of the main challenges regarding continuous and in vivo monitoring in complex
media like blood or human tissues. Based on recent findings, electrochemical sensors
offer one of the few routes to obtain continuous read-out and implantable devices
information referable to specific tissue locations [3]. In this regard, wearable devices
are at the forefront in both academic and industrial research on biosensors. The main
advantage for wearable technologies is the remote monitoring of human health by biomarkers
detection on the skin (e.g., continuous glucose self-monitoring in diabetic patients).
Nowadays, the minimally invasive collection of the sample implies the integration
of wearable biosensor platforms with microfluidic systems that allow information from
the sample to be transmitted directly from the skin to the electrode surface [4,5].
Moreover, the continuous and minimally invasive monitoring of biomarkers has also
become of fundamental importance in forensic, biometric, and cybersecurity fields.
McGoldrick et al. [6] reported on the possibility of using different bodily fluids
for metabolite analysis. This provides an alternative to the use of DNA in order to
avoid the backlog that is currently the main issue with DNA analysis by providing
worthwhile information about the originator.
Despite the efforts of the scientific community towards the development of minimally
invasive and wearable electrochemical biosensors for continuous and in vivo self-monitoring,
the fundamental theory behind electrochemical biosensors development still remains
a landmark. In particular for enzymes-based biosensors, most bioelectrochemists have
focused their attention on possible solutions to tackle direct electron transfer (DET)
issues, which are important for enhancing the selectivity and sensitivity of biosensors
[7]. Particular attention has been devoted to the case of glucose oxidase (GOx). Despite
the huge number of publications on this subject, which unfortunately account for thousands
of citations, there is no solid evidence to support DET in GOx, as demonstrated by
a stunning statement made by George Wilson: “based on recent experimental results,
the observed electrochemical signal corresponds to the FAD cofactor non-covalently
bound to the enzyme scaffold that comes out from the redox enzyme upon application
of potential, getting adsorbed onto the electrode surface” [8].
Beyond the use of GOx as a redox enzyme, there are several enzymes that are able to
transfer electrons according to direct or mediated pathways. In nature, many enzymes
are attached or inserted into a cell membrane, having hydrophobic subunits or lipid
chains for this purpose. Their reconstitution on electrodes allows them to maintain
their natural structural characteristics and enables the optimization of their electrocatalytic
properties and stability. In this regard, Alvarez-Malmagro et al. [9] discussed different
biomimetic strategies to modify electrode surfaces in order to accommodate membrane-bound
enzymes, including the formation of self-assembled monolayers of hydrophobic compounds,
lipid bilayers, or liposome deposition.
Besides the “classical” enzymes-based biosensors, in the last two decades, many enzymes
have been coupled with semiconductive electrodes containing a light-harvesting material
in order to develop photoelectrochemical sensing devices. Del Barrio et al. [10] reported
on the integration of nanomaterials, such as quantum dots and titanium oxide (TiO2)
nanoparticles with redox enzymes (e.g., acetylcholinesterase (AChE), glucose oxidase
(GOx), etc.), in order to enhance device sensitivity. Considering the successful results
in this specific field, future research trends will certainly involve the investigation
of different combinations of semiconductor materials and biomolecules and will also
consider the possibility of tuning the wavelength to develop a multi-analyte photoelectrochemical
biosensor. In particular, Neumann et al. [11] reported on the possibility of combining
artificial and natural heme peroxidases with semiconductive electrodes in order to
offer new read-out possibilities for hydrogen peroxide and phenolic compounds detection.
Moreover, the continuous and renown efforts toward the development of nanomaterial-modified
electrodes represent another aspect that has been deeply disclosed in the present
collection. In particular, Campuzano et al. [12] covered the topic of the modification
of electrode surfaces with antibiofouling reagents, which will eventually prevent
the non-specific adsorption of biological species on the electrode surface. This is
an important topic, especially considering the research on multiplexed and point-of-care
devices as cost-effective and selective multianalyte detection methods. Among all
the strategies currently available to develop antibiofouling surfaces, the modification
of electrode substrates with different biomaterials, including monolayers, transient
polymeric coatings, or multifunctional peptides, is particularly attractive and promising.
In this collection, the use of structured materials, such as nanoporous metals, graphene,
carbon nanotubes, and ordered mesoporous carbon, for biosensing applications has been
deeply discussed [13].
Recently, sulfur-containing nanomaterials and their derivatives/composites have been
extensively employed for the development of alternative biosensing devices. Li et
al. [14] summarized the recent findings and future challenges of employing metallic
sulfide nanomaterial-modified electrodes, particularly disclosing their specific properties,
namely, nanometric scale, water dispersibility, large specific surface area, excellent
catalytic activity, conductivity, biosafety, photoluminescence (PL) quenching abilities,
photoactivity, and fascinating optical properties.
Beyond graphene and graphene-like-2D-nanomaterials (e.g., sulfur-containing nanomaterials
etc.), Khan et al. [15] reported on the possibility of exploiting MXenes as 2D-layered
nanomaterials that provide unique capabilities for bioanalytical applications. These
include high metallic conductivity, large surface area, hydrophilicity, high ion transport
properties, low diffusion barrier, biocompatibility, and ease of surface functionalization.
Considering special features of nanomaterials, Stasyuk et al. [16] summarized the
recent findings about nanozymes. Nanozymes are defined as nanomaterials with enzyme-biomimicking
features (e.g., gold nanoparticles that mimic oxidases activity, etc.). This contribution
gives an overview of the classification of the nanozymes, their advantages vs. natural
enzymes, and their potential practical applications, devoting particular attention
to the different synthesis methods developed so far.
Beyond enzyme-based biosensors, immunosensors are also used for the development of
point-of-care devices. In particular, Sharafeldin et al. [17] reviewed the most recent
findings on 3D-printed immunosensing devices for cancer detection. In the last few
years, 3D-printing platforms have been used to produce complex sensor devices with
high resolution.
Moreover, aptasensors and DNA-modified electrodes have also been identified as point-of-care
devices that are especially useful for quick diagnostics during pandemic emergencies.
Santhanam et al. [18] summarized the most recent findings about DNA/RNA-based biosensors,
especially considering classical detection method pitfalls, such as for reverse transcription
PCR (RT-PCR) and real-time PCR (qPCR), which are considered time-consuming and require
specialized professionals and instrumentation.
On this specific topic, researchers are not focused only on the development of new
detection platforms, but they are also addressing potential issues about biosensor
sensitivity through different signal amplification methods. Smith et al. [19] reported
on recent findings and future challenges surrounding DNA detection based on a direct
restriction endonuclease (REase) assay. This assay allows for detection at an attomolar
level through an exponential signal amplification method based on a cascade of self-perpetuating
restriction endonuclease reactions, which induce continuous cleavage of amplification
probes, thus leading to exponential signal amplification. The proposed approach provides
a cost-, time-, and labor-effective alternative DNA detection method.
Besides the detection of DNA or antigens, immunosensors, DNA/RNA biosensors, and aptasensors
are currently considered in microbiology as powerful tools for the detection of bacteria
cells at a single cell level. These biosensors allow for the specific detection of
bacteria in complex biological matrices, often in the presence of excessive amounts
of other bacterial species [20].
The research within the biosensing field is not only focused on electrochemical and
optical transduction techniques but is also currently considering different approaches
to obtain a direct electronic read-out, like for electrolyte-insulator-semiconductor
(EIS) field-effect sensors, which belong to a new generation of electronic chips.
Poghossian and Schöning [21] gave an overview on recent advances and current trends
in the research and development of chemical sensors and biosensors based on the capacitive
field-effect EIS structure—the simplest field-effect device, which represents a biochemically
sensitive capacitor. Similarly, Sedki et al. [22] reported the most recent findings
on non-carbon 2D-materials-FET biosensors, discussing how transition metal dichalcogenides
(TMDCs), hexagonal boron nitride (h-BN), black phosphorus (BP), and metal oxides impacted
the development of the FET-based biosensors.