In celebration of receiving indexation from the Directory of Open Access Journals (DOAJ), we are pleased to introduce the collection, 2021 Retrospective: Frontiers in Sensors. This collection showcases well-received spontaneous articles since the Journal’s launch in mid-2020, which have been specially handpicked by our Chief Editor, Prof. Dermot Diamond. The work presented here highlights the broad diversity of research performed across the section, and aims to put a spotlight on the main areas of interest. All research presented here displays strong advances in theory, experiment and methodology with applications to compelling problems. This collection aims to further support Frontiers’ strong community by recognizing highly deserving authors.

Dublin City University

Frontiers in Sensors

Physical Sensors

Chemical Sensors

Sensor Devices

Electrochemical Sensors

Lab-on-a-Chip Devices

Biosensors

Micro- and Nano- Sensors

Sensor Networks

Optoelectronic and Photonic Sensors

2021 Retrospective: Frontiers in Sensors

Neurotransmitters, which are responsible for the signal transduction of nerve cells in the brain, are linked not only to various emotions and behaviors in our daily life, but also to brain diseases. Measuring neurotransmitters in the brain therefore makes a significant contribution to the progress of brain science. The purpose of this study is to develop a flexible thin film-type sensor that can electrochemically measure dopamine (DA) selectively and with high sensitivity. The thin-film sensor was prepared by printing gold colloidal ink on a polyimide film with a thickness of 25 µm—which the most flexible of the films examined that could maintain the buckling load (1 mN) required for insertion into the brain. The electrode (DA-PPy electrode) was then prepared by electropolymerization of polypyrrole (PPy) using DA as a template. The flexural rigidity of the sensor was 4.3 × 103 nNm, which is the lowest of any neurotransmitter sensors reported to date. When a DA solution (0–50 nM) was measured with the DA-PPy electrode using square-wave voltammetry (SWV), the slope of the calibration curve was 3.3 times higher than that of the PPy only negative control electrode, indicating an improvement in sensitivity by molecular imprinting with DA. The sensor was used to measure 0−50 nM norepinephrine (NE) and serotonin (5-HT), and the slope of the DA calibration curve at 0.24 V (19 ± 4.4 nA/nM) was much greater than those of NE (0.99 ± 3.3 nA/nM) and 5-HT (2.5 ± 2.4 nA/nM) because the selectivity for DA was also improved by molecular imprinting.

Neurotransmitters, which are responsible for the signal transduction of nerve cells in the brain, are linked not only to various emotions and behaviors in our daily life, but also to brain diseases. Measuring neurotransmitters in the brain therefore makes a significant contribution to the progress of brain science. The purpose of this study is to develop a flexible thin film-type sensor that can electrochemically measure dopamine (DA) selectively and with high sensitivity. The thin-film sensor was prepared by printing gold colloidal ink on a polyimide film with a thickness of 25&nbsp;µm—which the most flexible of the films examined that could maintain the buckling load (1&nbsp;mN) required for insertion into the brain. The electrode (DA-PPy electrode) was then prepared by electropolymerization of polypyrrole (PPy) using DA as a template. The flexural rigidity of the sensor was 4.3 × 103 nNm, which is the lowest of any neurotransmitter sensors reported to date. When a DA solution (0–50&nbsp;nM) was measured with the DA-PPy electrode using square-wave voltammetry (SWV), the slope of the calibration curve was 3.3 times higher than that of the PPy only negative control electrode, indicating an improvement in sensitivity by molecular imprinting with DA. The sensor was used to measure 0−50&nbsp;nM norepinephrine (NE) and serotonin (5-HT), and the slope of the DA calibration curve at 0.24&nbsp;V (19 ± 4.4&nbsp;nA/nM) was much greater than those of NE (0.99 ± 3.3&nbsp;nA/nM) and 5-HT (2.5 ± 2.4&nbsp;nA/nM) because the selectivity for DA was also improved by molecular imprinting.

Flexible Film-Type Sensor for Electrochemical Measurement of Dopamine Using a Molecular Imprinting Method

A portable sensing platform for the detection of nutrients (PO43−, NO2−, NO3−) in natural waters has been realized through the use of rapid prototyping techniques, colorimetric chemistries, electronics, and LED-based optical detection. The sensing platform is modular in design incorporating interchangeable optical detection units, with a component cost per unit of ca. €300, and small form factor (20 cm × 6 cm x 3.5 cm). Laboratory testing and validation of the platform was performed prior to deployment at the CNR Dirigibile Italia Arctic Research Station, Ny-Aselund (79°N, 12°E). Results obtained showed excellent linear response, with a limit of detection of 0.05 μM (NO2−, NO3−), and 0.03 μM (PO43−). On the June 22, 2016 a field campaign took place within Kongsfjorden, Ny-Aselund (78.5–79°N, 11.6–12.6°E), during which 55 water samples were acquired using 10 L Niskin bottles on board the MS Teisten research vessel. 23 hydrological casts were also performed using a Seabird 19plus V2 SeaCAT Profiler CTD probe with turbidity and dissolved oxygen sensors. Water samples were subsequently analyzed for PO43−, NO2−, NO3− at the CNR Dirigibile Italia Arctic Research Station Laboratory using the adaptive sensing platform. Nutrient concentrations were compared to hydrological data to assess the processes that influence the nutrient concentrations within the Fjord. This research highlights the potential use of the adaptive sensing platform in remote locations as a stand-alone platform and/or for the validation of deployable environmental sensor networks.

A portable sensing platform for the detection of nutrients (PO43−, NO2−, NO3−) in natural waters has been realized through the use of rapid prototyping techniques, colorimetric chemistries, electronics, and LED-based optical detection. The sensing platform is modular in design incorporating interchangeable optical detection units, with a component cost per unit of ca. €300, and small form factor (20&nbsp;cm × 6&nbsp;cm x 3.5&nbsp;cm). Laboratory testing and validation of the platform was performed prior to deployment at the CNR Dirigibile Italia Arctic Research Station, Ny-Aselund (79°N, 12°E). Results obtained showed excellent linear response, with a limit of detection of 0.05&nbsp;μM (NO2−, NO3−), and 0.03&nbsp;μM (PO43−). On the June 22, 2016 a field campaign took place within Kongsfjorden, Ny-Aselund (78.5–79°N, 11.6–12.6°E), during which 55 water samples were acquired using 10&nbsp;L Niskin bottles on board the MS Teisten research vessel. 23 hydrological casts were also performed using a Seabird 19plus V2 SeaCAT Profiler CTD probe with turbidity and dissolved oxygen sensors. Water samples were subsequently analyzed for PO43−, NO2−, NO3− at the CNR Dirigibile Italia Arctic Research Station Laboratory using the adaptive sensing platform. Nutrient concentrations were compared to hydrological data to assess the processes that influence the nutrient concentrations within the Fjord. This research highlights the potential use of the adaptive sensing platform in remote locations as a stand-alone platform and/or for the validation of deployable environmental sensor networks.

Nutrient Analysis in Arctic Waters Using a Portable Sensing Platform

A modified carbon fibre yarn sensor was developed for the voltammetric determination of paracetamol and its interferents (dopamine and ascorbic acid). Reduced graphene oxide (rGO) was electrochemically deposited onto a carbon fibre yarn. Further modification was achieved using polypyrrole (PPy) coated onto the rGO carbon fibre yarn via electropolymerisation of pyrrole with cyclic voltammetry (CV). The surface of the rGO and PPy-rGO carbon fibre electrodes were characterised using Raman spectroscopy and scanning electron microscopy. The rGO and PPy-rGO carbon fibres had a 3.5-fold and 7-fold larger electrochemical surface area compared to bare carbon fibre (calculated using the Randles-Sevcik equation). Two clearly distinguished oxidation peaks at 0.49 and 0.25 V (vs. Ag/AgCl) were observed at the rGO fibre electrode during the simultaneous detection of paracetamol and dopamine, respectively, by CV. The detection limit (3σ S/N) of the rGO carbon fibre electrode for differential pulse voltammetry (DPV) determination of paracetamol was at 21.1 and 6.0 µM for dopamine. In comparison, the simultaneous determination of paracetamol and dopamine by CV at the PPy-rGO fibre electrode gave oxidation peaks of paracetamol and dopamine at 0.55 and 0.25 V (vs. Ag/AgCl), respectively. The detection limit (3σ S/N) for paracetamol was notably improved to 3.7 µM and maintained at 6.0 µM for dopamine at the PPy-rGO carbon fibre electrode during DPV.

A modified carbon fibre yarn sensor was developed for the voltammetric determination of paracetamol and its interferents (dopamine and ascorbic acid). Reduced graphene oxide (rGO) was electrochemically deposited onto a carbon fibre yarn. Further modification was achieved using polypyrrole (PPy) coated onto the rGO carbon fibre yarn via electropolymerisation of pyrrole with cyclic voltammetry (CV). The surface of the rGO and PPy-rGO carbon fibre electrodes were characterised using Raman spectroscopy and scanning electron microscopy. The rGO and PPy-rGO carbon fibres had a 3.5-fold and 7-fold larger electrochemical surface area compared to bare carbon fibre (calculated using the Randles-Sevcik equation). Two clearly distinguished oxidation peaks at 0.49 and 0.25&#xa0;V (vs. Ag/AgCl) were observed at the rGO fibre electrode during the simultaneous detection of paracetamol and dopamine, respectively, by CV. The detection limit (3&#x3c3; S/N) of the rGO carbon fibre electrode for differential pulse voltammetry (DPV) determination of paracetamol was at 21.1 and 6.0&#xa0;&#xb5;M for dopamine. In comparison, the simultaneous determination of paracetamol and dopamine by CV at the PPy-rGO fibre electrode gave oxidation peaks of paracetamol and dopamine at 0.55 and 0.25&#xa0;V (vs. Ag/AgCl), respectively. The detection limit (3&#x3c3; S/N) for paracetamol was notably improved to 3.7&#xa0;&#xb5;M and maintained at 6.0&#xa0;&#xb5;M for dopamine at the PPy-rGO carbon fibre electrode during DPV.

Reduced Graphene Oxide Carbon Yarn Electrodes for Drug Sensing

Surface Acoustic Waves (SAW) sensors are known to be an excellent choice for the measurement of a small concentration of analytes in gas mixtures. The use of this type of sensor has been limited until now in the industrial environment due to the sensitivity of its response to temperature variations. To overcome this problem, thermal stabilization of equipment is normally used. We propose here a simple procedure of compensation of thermal drift in SAW sensors, allowing the measurements to be performed in temperature intervals of up to 20 degrees without any thermal stabilization of the sensitive element of a sensor. By monitoring the temperature of the key points of the sensor and applying the proposed polynomial compensation, it is possible to reduce the influence of thermal instabilities of the ambient temperature to the response more than four times. The method is illustrated by a temperature compensated SAW humidity sensor with a graphene oxide nanofilm as water molecules’ sensitive element. The results show enhanced performance of the sensor over a large temperature interval.

Surface Acoustic Waves (SAW) sensors are known to be an excellent choice for the measurement of a small concentration of analytes in gas mixtures. The use of this type of sensor has been limited until now in the industrial environment due to the sensitivity of its response to temperature variations. To overcome this problem, thermal stabilization of equipment is normally used. We propose here a simple procedure of compensation of thermal drift in SAW sensors, allowing the measurements to be performed in temperature intervals of up to 20 degrees without any thermal stabilization of the sensitive element of a sensor. By monitoring the temperature of the key points of the sensor and applying the proposed polynomial compensation, it is possible to reduce the influence of thermal instabilities of the ambient temperature to the response more than four times. The method is illustrated by a temperature compensated SAW humidity sensor with a graphene oxide nanofilm as water molecules&#x2019; sensitive element. The results show enhanced performance of the sensor over a large temperature interval.

Improved Stability and Performance of Surface Acoustic Wave Nanosensors Using a Digital Temperature Compensation

Herein, we demonstrate the prototype of a combined flexible pressure sensor based on ultrathin multiwall carbon nanotubes (MWCNTs) and graphite nanobelts (GNBs) films embedded in polydimethylsiloxane (PDMS). A simple and scalable modified Langmuir–Blodgett method was used for deposition of both MWCNT and GNB films. The use of two types of carbon nanostructures (nanotubes and GNBs) with distinctly different mechanical properties allowed obtaining enhanced dynamic range for pressure sensing. Short response time, good sensibility and flexibility, and low power consumption for enhanced pressure range make possible applications of the sensor for healthcare monitoring and as a component in the human–machine interfaces application.

Herein, we demonstrate the prototype of a combined flexible pressure sensor based on ultrathin multiwall carbon nanotubes (MWCNTs) and graphite nanobelts (GNBs) films embedded in polydimethylsiloxane (PDMS). A simple and scalable modified Langmuir&#x2013;Blodgett method was used for deposition of both MWCNT and GNB films. The use of two types of carbon nanostructures (nanotubes and GNBs) with distinctly different mechanical properties allowed obtaining enhanced dynamic range for pressure sensing. Short response time, good sensibility and flexibility, and low power consumption for enhanced pressure range make possible applications of the sensor for healthcare monitoring and as a component in the human&#x2013;machine interfaces application.

Combined Pressure Sensor With Enhanced Dynamic Range Based on Thin Films of Nanotubes and Graphite Nanobelts

With the advent of smart flexible electronic devices, new research directions have emerged. Among them, the resistive switching (RS) sensor has attracted much attention. The RS sensor converts the change of voltage signals into the change of resistance values. In this work, a planar flexible RS structure based on one-step printable silver electrodes was designed and fabricated to simplify device fabrication. The study is a starting point that paves the way for the development of all-in-one printable and flexible sensors in the future.

With the advent of smart flexible electronic devices, new research directions have emerged. Among them, the resistive switching (RS) sensor has attracted much attention. The RS sensor converts the change of voltage signals into the change of resistance values. In this work, a planar flexible RS structure based on one-step printable silver electrodes was designed and fabricated to simplify device fabrication. The study is a starting point that paves the way for the development of all-in-one printable and flexible sensors in the future.

Printable and Flexible Planar Silver Electrodes-Based Resistive Switching Sensory Array

The integration of functional synthetic materials and living biological entities has emerged as a new and powerful approach to create adaptive and functional structures with unprecedented performance and functionalities. Such hybrid structures are also called engineered living materials (ELMs). ELMs have the potential to realize many highly-desired properties, which are usually only found in biological systems, such as the abilities to self-power, self-heal, response to biosignals, and self-sustainable. Motivated by that, in recent years, researchers have started to explore the use of ELMs in many areas, among them, sensing and actuation is the area that has seen the most progress. In this short review, we briefly reviewed the important recent development in ELMs-based sensors and actuators, with a focus on their materials and structural design, new fabrication technologies, and bio-related applications. Current challenges and future directions in this field are also identified to help with future development in this emerging interdisciplinary field.

The integration of functional synthetic materials and living biological entities has emerged as a new and powerful approach to create adaptive and functional structures with unprecedented performance and functionalities. Such hybrid structures are also called engineered living materials (ELMs). ELMs have the potential to realize many highly-desired properties, which are usually only found in biological systems, such as the abilities to self-power, self-heal, response to biosignals, and self-sustainable. Motivated by that, in recent years, researchers have started to explore the use of ELMs in many areas, among them, sensing and actuation is the area that has seen the most progress. In this short review, we briefly reviewed the important recent development in ELMs-based sensors and actuators, with a focus on their materials and structural design, new fabrication technologies, and bio-related applications. Current challenges and future directions in this field are also identified to help with future development in this emerging interdisciplinary field.