A Microfluidic Love-Wave Biosensing Device for PSA Detection Based on an Aptamer Beacon Probe

Finite Element Study for Mass Sensitivity of Love Surface Acoustic Wave Sensor with Si3N4-SiO2 Double-Covered Waveguiding Layer

Love surface acoustic wave (L-SAW) sensors are miniaturized, easy to integrate, and suitable for detection in liquid environments. In this paper, an L-SAW sensor with a thin Si3N4-SiO2 double-covered layer was proposed for samples with small mass loads. The output response, phase velocity of the acoustic wave, and the mass sensitivity were analyzed using the finite element method (FEM). The simulation results show that the Si3N4 layer with high wave velocity greatly weakens the limitation of SiO2 on the phase velocity. The phase velocity can reach about 4300 m/s, which can increase the frequency shift when the same mass load is applied. Within a certain range, the mass sensitivity of the sensor is enhanced with the increase in the total thickness of the waveguiding layer and the thickness ratio of Si3N4 in the double-covered layer. When the thickness ratio is 1:2, the peak value of the mass sensitivity of the sensor is approximately 50% higher than that achieved with only the SiO2 waveguiding layer. The surface average stress of the delay line region follows the same trend as the mass sensitivity. The increase in mass sensitivity is the result of the heightened stress on the sensor surface. This L-SAW sensor, featuring a double-covered waveguiding layer, demonstrates high sensitivity and a simple structure. The simulation results lay a foundation for the design and manufacture of SAW sensors.

Liquid Viscosity Sensor Using a Surface Acoustic Wave Device for Medical Applications Including Blood and Plasma

Blood viscosity is the defining health indicator for hyperviscosity syndrome patients. This paper introduces an alternative approach for the real-time monitoring of blood viscosity by employing a surface-horizontal surface acoustic wave (SH-SAW) device at room temperature. A novel bi-layer waveguide is constructed on top of the SAW device. This device enables the SAW sensing of liquid droplets utilizing a bi-layer waveguide, consisting of a zinc oxide (ZnO) enhancement layer and Parlyene C, that facilitates the promotion of the surface horizontal mode. The ZnO piezoelectric thin-film layer enhanced the local particle displacement and dielectric coupling while the Parylene C layer constrained the wave mode at the interface of the piezoelectric material and polymer material. The device was tested with a liquid drop on the SAW delay-line path. Both experimental and finite element analysis results demonstrated the benefits of the bi-layer waveguide. The simulation results confirmed that the displacement field of local particles increased 9 times from 1.261 nm to 11.353 nm with the Parylene C/ZnO bi-layer waveguide structure. The device demonstrated a sensitivity of 3.57 ± 0.3125 kHz shift per centipoise enabling the potential for high precision blood viscosity monitoring.

A Study of Optimizing Lamb Wave Acoustic Mass Sensors' Performance through Adjustment of the Transduction Electrode Metallization Ratio

This paper presents the design and simulation of a mass sensitive Lamb wave microsensor with CMOS technology provided by SilTerra. In this work, the effects of the metalization ratio variation on the transmission gain, total harmonic distortion (THD), and two different resonant modes (around 66 MHz and 86 MHz) are shown. It has been found that the metalization ratio can be adjusted in order to obtain a compromise between transmission gain and sensitivity, depending on the design criteria. By adding a Si3N4 layer on top of the device, a five-fold improvement in transmission gain is reached. It was also shown that the transmission of the input differential IDT configuration is 20% more efficient than a single terminal. With this combination, the mass sensitivity is about 114 [cm2/gr].

Acoustic Biosensors and Microfluidic Devices in the Decennium: Principles and Applications

Lab-on-a-chip (LOC) technology has gained primary attention in the past decade, where label-free biosensors and microfluidic actuation platforms are integrated to realize such LOC devices. Among the multitude of technologies that enables the successful integration of these two features, the piezoelectric acoustic wave method is best suited for handling biological samples due to biocompatibility, label-free and non-invasive properties. In this review paper, we present a study on the use of acoustic waves generated by piezoelectric materials in the area of label-free biosensors and microfluidic actuation towards the realization of LOC and POC devices. The categorization of acoustic wave technology into the bulk acoustic wave and surface acoustic wave has been considered with the inclusion of biological sample sensing and manipulation applications. This paper presents an approach with a comprehensive study on the fundamental operating principles of acoustic waves in biosensing and microfluidic actuation, acoustic wave modes suitable for sensing and actuation, piezoelectric materials used for acoustic wave generation, fabrication methods, and challenges in the use of acoustic wave modes in biosensing. Recent developments in the past decade, in various sensing potentialities of acoustic waves in a myriad of applications, including sensing of proteins, disease biomarkers, DNA, pathogenic microorganisms, acoustofluidic manipulation, and the sorting of biological samples such as cells, have been given primary focus. An insight into the future perspectives of real-time, label-free, and portable LOC devices utilizing acoustic waves is also presented. The developments in the field of thin-film piezoelectric materials, with the possibility of integrating sensing and actuation on a single platform utilizing the reversible property of smart piezoelectric materials, provide a step forward in the realization of monolithic integrated LOC and POC devices. Finally, the present paper highlights the key benefits and challenges in terms of commercialization, in the field of acoustic wave-based biosensors and actuation platforms.

Numerical and experimental analysis of a hybrid material acoustophoretic device for manipulation of microparticles

Acoustophoretic microfluidic devices have been developed for accurate, label-free, contactless, and non-invasive manipulation of bioparticles in different biofluids. However, their widespread application is limited due to the need for the use of high quality microchannels made of materials with high specific acoustic impedances relative to the fluid (e.g., silicon or glass with small damping coefficient), manufactured by complex and expensive microfabrication processes. Soft polymers with a lower fabrication cost have been introduced to address the challenges of silicon- or glass-based acoustophoretic microfluidic systems. However, due to their small acoustic impedance, their efficacy for particle manipulation is shown to be limited. Here, we developed a new acoustophoretic microfluid system fabricated by a hybrid sound-hard (aluminum) and sound-soft (polydimethylsiloxane polymer) material. The performance of this hybrid device for manipulation of bead particles and cells was compared to the acoustophoretic devices made of acoustically hard materials. The results show that particles and cells in the hybrid material microchannel travel to a nodal plane with a much smaller energy density than conventional acoustic-hard devices but greater than polymeric microfluidic chips. Against conventional acoustic-hard chips, the nodal line in the hybrid microchannel could be easily tuned to be placed in an off-center position by changing the frequency, effective for particle separation from a host fluid in parallel flow stream models. It is also shown that the hybrid acoustophoretic device deals with smaller temperature rise which is safer for the actuation of bioparticles. This new device eliminates the limitations of each sound-soft and sound-hard materials in terms of cost, adjusting the position of nodal plane, temperature rise, fragility, production cost and disposability, making it desirable for developing the next generation of economically viable acoustophoretic products for ultrasound particle manipulation in bioengineering applications.

The Application of Microfluidic Technologies in Aptamer Selection

Aptamers are sequences of single-strand oligonucleotides (DNA or RNA) with potential binding capability to specific target molecules, which are increasingly used as agents for analysis, diagnosis, and medical treatment. Aptamers are generated by a selection method named systematic evolution of ligands by exponential enrichment (SELEX). Numerous SELEX methods have been developed for aptamer selections. However, the conventional SELEX methods still suffer from high labor intensity, low operation efficiency, and low success rate. Thus, the applications of aptamer with desired properties are limited. With their advantages of low cost, high speed, and upgraded extent of automation, microfluidic technologies have become promising tools for rapid and high throughput aptamer selection. This paper reviews current progresses of such microfluidic systems for aptamer selection. Comparisons of selection performances with discussions on principles, structure, operations, as well as advantages and limitations of various microfluidic-based aptamer selection methods are provided.

A label-free fluorescent aptasensor based on HCR and G-quadruplex DNAzymes for the detection of prostate-specific antigen

Prostate specific antigen (PSA) has been considered as the most potential serological biomarker for the early stage detection of prostate cancer. Here, a label-free fluorescence aptasensing strategy for detecting PSA based on hybridization chain reaction (HCR) and G-quadruplex DNAzymes has been developed. This designed strategy consists of three DNA probes, aptamer probe (AP), hairpin probe 1 (H1) and hairpin probe 2 (H2). In the presence of target PSA, the aptamer sequences in AP specifically recognized PSA to form a PSA-aptamer complex, causing an AP conformation change and thus releasing the initiator, which triggered the chain-like assembly of H1 and H2 that yielded extended nicked double-stranded DNA through HCR. Upon the addition of hemin, the G-rich segments at the end of H1 and H2 self-assembled into the peroxidase-mimicking hemin/G-quadruplex DNAzymes, which catalyzed the hydrogen peroxide-mediated oxidation of thiamine to give a fluorescence signal dependent on the concentration of PSA. Under optimal conditions, a limit of detection of 0.05 nM and a linear range from 0.1 nM to 1 nM (R2 = 0.9942) were achieved by this assay. In addition, other interfering proteins, such as IgG, AFP and CEA, did not produce any significant change in the fluorescence intensity response, indicating good selectivity of this sensor for PSA detection. Finally, this proposed aptasensor was successfully used for diluted serum samples.

Prostate cancer and microfluids

Microfluidic systems aim to detect sample matter quickly with high sensitivity and resolution, on a small scale. With its increased use in medicine, the field is showing significant promise in prostate cancer diagnosis and management due, in part, to its ability to offer point-of-care testing. This review highlights some of the research that has been undertaken in respect of prostate cancer and microfluidics. Firstly, this review considers the diagnosis of prostate cancer through use of microfluidic systems and analyses the detection of prostate specific antigen, proteins, and circulating tumor cells to highlight the scope of current advancements. Secondly, this review analyses progressions in the understanding of prostate cancer physiology and considers techniques used to aid treatment of prostate cancer, such as the creation of a micro-environment. Finally, this review highlights potential future roles of microfluidics in assisting prostate cancer, such as in exosomal analysis. In conclusion, this review shows the vast scope and application of microfluidic systems and how these systems will ensure advancements to future prostate cancer management.

Aptamers used for biosensors and targeted therapy

Aptamers are single-stranded nucleic acid sequences that can bind to target molecules with high selectivity and affinity. Most aptamers are screened in vitro by a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX). Since aptamers were discovered in the 1990s, they have attracted considerable attention and have been widely used in many fields owing to their unique advantages. In this review, we present an overview of the advancements made in aptamers used for biosensors and targeted therapy. For the former, we will discuss multiple aptamer-based biosensors with different principles detected by various signaling methods. For the latter, we will focus on aptamer-based targeted therapy using aptamers as both biotechnological tools for targeted drug delivery and as targeted therapeutic agents. Finally, challenges and new perspectives associated with these two regions were further discussed. We hope that this review will help researchers interested in aptamer-related biosensing and targeted therapy research.

Love wave based portable sensing system for on-line detection of carcinoembryonic antigen in exhaled breath condensate

Lung cancer is continuously the leading cause of cancer related death. Noninvasive exhaled breath condensate (EBC) detection using portable devices may pave a new avenue for monitoring conditions of the high-risk population of lung cancer, thereby increasing the 5-year survival rate. Here, a portable system is proposed for on-line detection of carcinoembryonic antigen (CEA) in EBC. This miniaturized system employs an aptameric Love-wave sensor to provide a detection with advantages of low cost, operational simplicity and high sensitivity. In addition, the aptasensor can be used in label-free and real-time monitoring. Integrating with measurement circuits and iOS APP, the system realizes online transmission and processing of signals which reflect the CEA concentration. Artificial samples have been detected, and experimental results demonstrate that the present system is sensitive (limit of detection = 1 ng/ml) and accurate (relative accuracy = 0.995). Applying in detection of clinical EBC, the relative error is as low as 0.305% and the result can be used to distinguish lung cancer patients from healthy subjects. Therefore, this portable system offers a potential solution for noninvasive detection of lung cancer in its early stage.

Pivotal role of electrospun nanofibers in microfluidic diagnostic systems - a review

Recently, the usage of electrospinning technology for the fabrication of fine fibers with a good deal of variation in morphology and structure has drawn the attention of many researchers around the world. These fibers have found their way in the many fields of science including medical diagnosis, tissue engineering, drug delivery, replica molding, solar cells, catalysts, energy conversion and storage, physical and chemical sensors and other applications. Among all applications, biosensing with the aim of rapid and sensitive biomarker detection is an area that warrants attention. Electrospun nanofibrous membranes enjoy numerous factors which benefit them to be used as potential candidates in biosensing platforms. Some of these factors include a high surface to volume ratio, analogous scale compared to bioactive molecules and relatively defect-free properties of nanofibers (NFs). In this review, we focused on the recent advances in electrospun nanofibrous membrane-based micro-analytical devices with an application as diagnostic systems. Hence, a study on the electrospun nanofiber usage in lab-on-a-chip and paper-based point-of-care devices, with an opening introduction to biosensors, nanofibers, the electrospinning method, and microfluidics as the principles of the intended subject, is provided. It is anticipated that the given examples in this paper will provide sufficient evidence for the potential of electrospun NFs for being used as a substrate in the commercial fabrication of highly sensitive and selective biosensors.

Lab-on-a-Chip Systems for Aptamer-Based Biosensing

Aptamers are oligonucleotides or peptides that are selected from a pool of random sequences that exhibit high affinity toward a specific biomolecular species of interest. Therefore, they are ideal for use as recognition elements and ligands for binding to the target. In recent years, aptamers have gained a great deal of attention in the field of biosensing as the next-generation target receptors that could potentially replace the functions of antibodies. Consequently, it is increasingly becoming popular to integrate aptamers into a variety of sensing platforms to enhance specificity and selectivity in analyte detection. Simultaneously, as the fields of lab-on-a-chip (LOC) technology, point-of-care (POC) diagnostics, and personal medicine become topics of great interest, integration of such aptamer-based sensors with LOC devices are showing promising results as evidenced by the recent growth of literature in this area. The focus of this review article is to highlight the recent progress in aptamer-based biosensor development with emphasis on the integration between aptamers and the various forms of LOC devices including microfluidic chips and paper-based microfluidics. As aptamers are extremely versatile in terms of their utilization in different detection principles, a broad range of techniques are covered including electrochemical, optical, colorimetric, and gravimetric sensing as well as surface acoustics waves and transistor-based detection.

Advances in prostate specific antigen biosensors-impact of nanotechnology

Prostate cancer is one of the most dangerous and deadly cancers in elderly men. Early diagnosis using prostate-specific antigen (PSA) facilitates disease detection, management and treatment. Biosensors have recently been used as sensitive, selective, inexpensive and rapid diagnostic tools for PSA detection. In this review, a variety of PSA biosensors such as aptasensors, peptisensors and immunesensors are highlighted. These use aptamers, peptides and antibodies in the biorecognition element, respectively, and can detect PSA with very high sensitivity via electrochemical, electrochemiluminescence, fluorescence and surface-enhanced Raman spectroscopy. To improve the sensitivity of most of these PSA biosensors, different nanostructured materials have played a critical role.

An aptamer-based shear horizontal surface acoustic wave biosensor with a CVD-grown single-layered graphene film for high-sensitivity detection of a label-free endotoxin

The thickness of the sensitive layer has an important influence on the sensitivity of a shear horizontal surface acoustic wave (SH-SAW) biosensor with a delay-line structure and lower layer numbers of graphene produce better sensitivity for biological detection. Therefore, a label-free and highly sensitive SH-SAW biosensor with chemical vapor deposition (CVD-)-grown single-layered graphene (SLG) for endotoxin detection was developed in this study. With this methodology, SH-SAW biosensors were fabricated on a 36° Y-90° X quartz substrate with a base frequency of 246.2 MHz, and an effective detection cell was fabricated using acrylic material. To increase the surface hydrophilicity, chitosan was applied to the surface of the SLG film. Additionally, the aptamer was immobilized on the surface of the SLG film by cross-linking with glutaraldehyde. Finally, the sensitivity was verified by endotoxin detection with a linear detection ranging from 0 to 100 ng/mL, and the detection limit (LOD) was as low as 3.53 ng/mL. In addition, the stability of this type of SH-SAW biosensor from the air phase to the liquid phase proved to be excellent and the specificity was tested and verified by detecting the endotoxin obtained from Escherichia coli (E. coli), the endotoxin obtained from Pseudomonas aeruginosa (P. aeruginosa), and aflatoxin. Therefore, this type of SH-SAW biosensor with a CVD-grown SLG film may offer a promising approach to endotoxin detection, and it may have great potential in clinical applications.

Recent Advances in Aptamer Discovery and Applications

Aptamers are short, single-stranded DNA, RNA, or synthetic XNA molecules that can be developed with high affinity and specificity to interact with any desired targets. They have been widely used in facilitating discoveries in basic research, ensuring food safety and monitoring the environment. Furthermore, aptamers play promising roles as clinical diagnostics and therapeutic agents. This review provides update on the recent advances in this rapidly progressing field of research with particular emphasis on generation of aptamers and their applications in biosensing, biotechnology and medicine. The limitations and future directions of aptamers in target specific delivery and real-time detection are also discussed.

Recent Microdevice-Based Aptamer Sensors

Since the systematic evolution of ligands by exponential enrichment (SELEX) method was developed, aptamers have made significant contributions as bio-recognition sensors. Microdevice systems allow for low reagent consumption, high-throughput of samples, and disposability. Due to these advantages, there has been an increasing demand to develop microfluidic-based aptasensors for analytical technique applications. This review introduces the principal concepts of aptasensors and then presents some advanced applications of microdevice-based aptasensors on several platforms. Highly sensitive detection techniques, such as electrochemical and optical detection, have been integrated into lab-on-a-chip devices and researchers have moved towards the goal of establishing point-of-care diagnoses for target analyses.

Surface Acoustic Wave (SAW) for Chemical Sensing Applications of Recognition Layers

Surface acoustic wave (SAW) resonators represent some of the most prominent acoustic devices for chemical sensing applications. As their frequency ranges from several hundred MHz to GHz, therefore they can record remarkably diminutive frequency shifts resulting from exceptionally small mass loadings. Their miniaturized design, high thermal stability and possibility of wireless integration make these devices highly competitive. Owing to these special characteristics, they are widely accepted as smart transducers that can be combined with a variety of recognition layers based on host-guest interactions, metal oxide coatings, carbon nanotubes, graphene sheets, functional polymers and biological receptors. As a result of this, there is a broad spectrum of SAW sensors, i.e., having sensing applications ranging from small gas molecules to large bio-analytes or even whole cell structures. This review shall cover from the fundamentals to modern design developments in SAW devices with respect to interfacial receptor coatings for exemplary sensor applications. The related problems and their possible solutions shall also be covered, with a focus on emerging trends and future opportunities for making SAW as established sensing technology.

A signal-on built in-marker electrochemical aptasensor for human prostate-specific antigen based on a hairbrush-like gold nanostructure

A green electrodeposition method was firstly employed for the synthesis of round hairbrush-like gold nanostructure in the presence of cadaverine as a size and shape directing additive. The nanostructure which comprised of arrays of nanospindles was then applied as a transducer to fabricate a signal-on built in-marker electrochemical aptasensor for the detection of human prostate-specific antigen (PSA). The aptasensor detected PSA with a linear concentration range of 0.125 to 128 ng mL-1 and a limit of detection of 50 pg mL-1. The aptasensor was then successfully applied to detect PSA in the blood serum samples of healthy and patient persons.

Nanomaterial-based biosensors for detection of prostate specific antigen

Screening serum for the presence of prostate specific antigen (PSA) belongs to the most common approach for the detection of prostate cancer. This review (with 156 refs.) addresses recent developments in PSA detection based on the use of various kinds of nanomaterials. It starts with an introduction into the field, the significance of testing for PSA, and on current limitations. A first main section treats electrochemical biosensors for PSA, with subsections on methods based on the use of gold electrodes, graphene or graphene-oxide, carbon nanotubes, hybrid nanoparticles, and other types of nanoparticles. It also covers electrochemical methods based on the enzyme-like activity of PSA, on DNA-, aptamer- and biofuel cell-based methods, and on the detection of PSA via its glycan part. The next main section covers optical biosensors, with subsections on methods making use of surface plasmon resonance (SPR), localized SPR and plasmonic ELISA-like schemes. This is followed by subsections on methods based on the use of fiber optics, fluorescence, chemiluminescence, Raman scattering and SERS, electrochemiluminescence and cantilever-based methods. The most sensitive biosensors are the electrochemical ones, with lowest limits of detection (down to attomolar concentrations), followed by mass cantilever sensing and electrochemilumenescent strategies. Optical biosensors show lower performance, but are still more sensitive compared to standard ELISA. The most commonly applied nanomaterials are metal and carbon-based ones and their hybrid composites used for different amplification strategies. The most attractive sensing schemes are summarized in a Table. The review ends with a section on conclusions and perspectives.

Visible-light driven Photoelectrochemical Immunosensor Based on SnS2@mpg-C3N4 for Detection of Prostate Specific Antigen

Herein, a novel label-free photoelectrochemical (PEC) immunosensor based on SnS2@mpg-C3N4 nanocomposite is fabricated for the detection of prostate specific antigen (PSA) in human serum. Firstly, mesoporous graphite-like carbon nitride (mpg-C3N4) with carboxyl groups is synthesized successfully which possesses high specific surface area and large pore volume. Then, SnS2 as a typical n-type semiconductor with weak photoelectric conversion capability is successfully loaded on carboxylated mpg-C3N4 to form a well-matched overlapping band-structure. The as-synthesized SnS2@mpg-C3N4 nanocomposite performs outstanding photocurrent response under visible-light irradiation due to low recombination rate of photoexcited electron-hole pairs, which is transcend than pure SnS2 or pure mpg-C3N4. It is worth noting that SnS2@mpg-C3N4 nanocomposite is firstly employed as the photoactive material in PEC immunosensor area. The concentration of PSA can be analyzed by the decrease in photocurrent resulted from increased steric hindrance of the immunocomplex. Under the optimal conditions, the developed PEC immunosensor displays a liner photocurrent response in the range of 50 fg·mL-1 ~ 10 ng·mL-1 with a low detection limit of 21 fg·mL-1. Furthermore, the fabricated immunosensor with satisfactory stability, reproducibility and selectivity provides a novel method for PSA determination in real sample analysis.

Theoretical Study of Monolayer and Double-Layer Waveguide Love Wave Sensors for Achieving High Sensitivity

Love wave sensors have been widely used for sensing applications. In this work, we introduce the theoretical analysis of the monolayer and double-layer waveguide Love wave sensors. The velocity, particle displacement and energy distribution of Love waves were analyzed. Using the variations of the energy repartition, the sensitivity coefficients of Love wave sensors were calculated. To achieve a higher sensitivity coefficient, a thin gold layer was added as the second waveguide on top of the silicon dioxide (SiO₂) waveguide-based, 36 degree-rotated, Y-cut, X-propagating lithium tantalate (36° YX LiTaO₃) Love wave sensor. The Love wave velocity was significantly reduced by the added gold layer, and the flow of wave energy into the waveguide layer from the substrate was enhanced. By using the double-layer structure, almost a 72-fold enhancement in the sensitivity coefficient was achieved compared to the monolayer structure. Additionally, the thickness of the SiO₂ layer was also reduced with the application of the gold layer, resulting in easier device fabrication. This study allows for the possibility of designing and realizing robust Love wave sensors with high sensitivity and a low limit of detection.

Microfluidics in the selection of affinity reagents for the detection of cancer: paving a way towards future diagnostics

Microfluidic technologies have miniaturized a variety of biomedical applications, and these chip-based systems have several significant advantages over their large-scale counterparts. Recently, this technology has been used for automating labor-intensive and time-consuming screening processes, whereby affinity reagents, including aptamers, peptides, antibodies, polysaccharides, glycoproteins, and a variety of small molecules, are used to probe for molecular biomarkers. When compared to conventional methods, the microfluidic approaches are faster, more compact, require considerably smaller quantities of samples and reagents, and can be automated. Furthermore, they allow for more precise control of reaction conditions (e.g., pH, temperature, and shearing forces) such that more efficient screening can be performed. A variety of affinity reagents for targeting cancer cells or cancer biomarkers are now available and will likely replace conventional antibodies. In this review article, the selection of affinity reagents for cancer cells or cancer biomarkers on microfluidic platforms is reviewed with the aim of highlighting the utility of such approaches in cancer diagnostics.

Aptamer-based 'point-of-care testing'

Aptamers are single-stranded oligonucleotides that can be artificially generated by a method called Systematic evolution of ligands by exponential enrichment (SELEX). The generated aptamers have been assessed for high-performance sensing applications due to their appealing characteristics. With either aptamers alone or complementing with antibodies, several high sensitive and portable sensors have been demonstrated for use in 'point-of-care testing'. Due to their high suitability and flexibility, aptamers are conjugated with nanostructures and utilized in field applications. Moreover, aptamers are more amenable to chemical modifications, making them capable of utilization with most developed sensors. In this overview, we discuss novel, portable, and aptamer-based sensing strategies that are suitable for 'point-of-care testing'.

Aptamers: versatile molecular recognition probes for cancer detection

In the past two decades, aptamers have emerged as a novel class of molecular recognition probes comprising uniquely-folded short RNA or single-stranded DNA oligonucleotides that bind to their cognate targets with high specificity and affinity. Aptamers, often referred to as "chemical antibodies", possess several highly desirable features for clinical use. They can be chemically synthesized and are easily conjugated to a wide range of reporters for different applications, and are able to rapidly penetrate tissues. These advantages significantly enhance their clinical applicability, and render them excellent alternatives to antibody-based probes in cancer diagnostics and therapeutics. Aptamer probes based on fluorescence, colorimetry, magnetism, electrochemistry, and in conjunction with nanomaterials (e.g., nanoparticles, quantum dots, single-walled carbon nanotubes, and magnetic nanoparticles) have provided novel ultrasensitive cancer diagnostic strategies and assays. Furthermore, promising aptamer targeted-multimodal tumor imaging probes have been recently developed in conjunction with fluorescence, positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). The capabilities of the aptamer-based platforms described herein underscore the great potential they hold for the future of cancer detection. In this review, we highlight the most prominent recent developments in this rapidly advancing field.