Single-walled carbon nanotubes chemiresistor aptasensors for small molecules: picomolar level detection of adenosine triphosphate

Carbon Nanotube (CNT)-Based Biosensors

This review focuses on recent advances in the application of carbon nanotubes (CNTs) for the development of sensors and biosensors. The paper discusses various configurations of these devices, including their integration in analytical devices. Carbon nanotube-based sensors have been developed for a broad range of applications including electrochemical sensors for food safety, optical sensors for heavy metal detection, and field-effect devices for virus detection. However, as yet there are only a few examples of carbon nanotube-based sensors that have reached the marketplace. Challenges still hamper the real-world application of carbon nanotube-based sensors, primarily, the integration of carbon nanotube sensing elements into analytical devices and fabrication on an industrial scale.

Comparison of Duplex and Quadruplex Folding Structure Adenosine Aptamers for Carbon Nanotube Field Effect Transistor Aptasensors

Carbon nanotube field effect transistor (CNT FET) aptasensors have been investigated for the detection of adenosine using two different aptamer sequences, a 35-mer and a 27-mer. We found limits of detection for adenosine of 100 pM and 320 nM for the 35-mer and 27-mer aptamers, with dissociation constants of 1.2 nM and 160 nM, respectively. Upon analyte recognition the 35-mer adenosine aptamer adopts a compact G-quadruplex structure while the 27-mer adenosine aptamer changes to a folded duplex. Using the CNT FET aptasensor platform adenosine could be detected with high sensitivity over the range of 100 pM to 10 µM, highlighting the suitability of the CNT FET aptasensor platform for high performance adenosine detection. The aptamer restructuring format is critical for high sensitivity with the G-quadraplex aptasensor having a 130-fold smaller dissociation constant than the duplex forming aptasensor.

Label-free chemiresistor biosensor based on reduced graphene oxide and M13 bacteriophage for detection of coliforms

Coliform bacteria are well known as informative indicators for bacterial contamination in water. This study presents a novel chemiresistor biosensor using M13 phage-modified reduced graphene oxide (rGO) for detection of Escherichia coli (E. coli), as coliform bacteria. M13 phage, as a biorecognition element, was immobilized on the rGO channel, so that it can bind to negatively charged E. coli bacteria, allowing the gating effect on the biosensor and the change in its resistance. The prepared materials and device were characterized using spectroscopic, microscopic, and electrical measurements. FTIR and XRD results proved the successful fabrication of GO and rGO nanosheets. AFM results showed that the prepared nanosheets were monolayer. The SEM micrographs of the M13-functionalized devices, soaked in two different concentrations of E. coli, confirmed the successful capturing of E. coli and that the signal change is concentration-dependent. As a result, a linear and specific response towards E. coli was observed and the limit of detection was determined to be 45 CFU/mL. Further, the proposed sensor system showed selectivity towards the tested coliforms. These results suggested this sensing system could be a promising tool for detecting coliforms with an economic, accurate, rapid, and directly applicable process.

Recent Developments in Aptasensors for Diagnostic Applications

Aptamers are exciting smart molecular probes for specific recognition of disease biomarkers. A number of strategies have been developed to convert target-aptamer binding into physically detectable signals. Since the aptamer sequence was first discovered, a large variety of aptamer-based biosensors have been developed, with considerable attention paid to their potential applications in clinical diagnostics. So far, a variety of techniques in combination with a wide range of functional nanomaterials have been used for the design of aptasensors to further improve the sensitivity and detection limit of target determination. In this paper, the advantages of aptamers over traditional antibodies as the molecular recognition components in biosensors for high-throughput screening target molecules are highlighted. Aptamer-target pairing configurations are predominantly single- or dual-site binding; the design of recognition modes of each aptamer-target pairing configuration is described. Furthermore, signal transduction strategies including optical, electrical, mechanical, and mass-sensitive modes are clearly explained together with examples. Finally, we summarize the recent progress in the development of aptamer-based biosensors for clinical diagnosis, including detection of cancer and disease biomarkers and in vivo molecular imaging. We then conclude with a discussion on the advanced development and challenges of aptasensors.

Predicting Future Prospects of Aptamers in Field-Effect Transistor Biosensors

Aptamers, in sensing technology, are famous for their role as receptors in versatile applications due to their high specificity and selectivity to a wide range of targets including proteins, small molecules, oligonucleotides, metal ions, viruses, and cells. The outburst of field-effect transistors provides a label-free detection and ultra-sensitive technique with significantly improved results in terms of detection of substances. However, their combination in this field is challenged by several factors. Recent advances in the discovery of aptamers and studies of Field-Effect Transistor (FET) aptasensors overcome these limitations and potentially expand the dominance of aptamers in the biosensor market.

Affinity sensor for haemoglobin A1c based on single-walled carbon nanotube field-effect transistor and fructosyl amino acid binding protein

Haemoglobin A1c (HbA1c) is a significant glycaemic marker for diabetes mellitus. The level of HbA1c reflects the mean blood glucose level over the prior 2-3 months and it is useful for the assessment of therapeutic effectiveness and for diagnosis. In this study, we report the label-free affinity sensor for HbA1c based on the chemiresistor-type field-effect transistor, which has a simple sensor configuration. Single-walled carbon nanotubes (SWNTs) were used as the transducing element. The fructosyl amino acid binding protein from Rhizobium radiobacter (SocA), which binds to α-fructosyl amino acid specifically, was used as the biorecognition element for fructosyl valine (FV), the product of the proteolytic hydrolysis of HbA1c. The developed sensor shows the ability to measure as low as 1.2 nM FV, which is 14-fold more sensitive compared to the previously reported fluorescence-based sensor using SocA. This sensor also exhibits high specificity where no significant response is observed from either fructosyl lysine (FK) or glucose, which are potential interferents. FK is the ε-fructosyl amino acid from glycated albumin, another glycated protein, whereas glucose is naturally present at very high concentration in the blood. We propose that the modulation of the surface charges on the SWNTs caused by the conformational change in SocA upon ligand binding leads to the proportionate changes in the number of carriers in the SWNT channel.

A fluorescence aptasensor based on two-dimensional sheet metal-organic frameworks for monitoring adenosine triphosphate

In the present study, a facile fluorescence aptasensor based on two-dimensional sheet metal-organic frameworks of N,N-bis(2-hydroxyethyl)dithiooxamidato copper(II) (H₂dtoaCu) was developed for the sensitive detection of adenosine triphosphate (ATP). The sensing mechanism was based on the noncovalent interaction between FAM-labeled (fluorescein amidite) ATP aptamers and H₂dtoaCu. In the absence of ATP, the FAM-labeled aptamer readily adsorbs onto H₂dtoaCu, mainly via π-π stacking and hydrogen bond interactions between the nucleotide bases and the H₂dtoaCu surface, leading to the reduction of fluorescence intensity of the FAM by photoinduced electron transfer (PET). In the presence of ATP, the FAM-labeled aptamer specifically forms ATP-binding aptamer complexes which exhibit only weak adsorption on the H₂dtoaCu surface. Thus, the fluorescence of the FAM-labeled ATP aptamer remained largely unchanged. The fluorescence aptasensor exhibited a good linear relationship between the fluorescence intensity and the logarithm concentration of ATP over a range of 25-400 nM, with a detection limit of 8.19 nM (3S/N). ATP analogs such as guanosine triphosphate, uridine triphosphate, and cytidine triphosphate have negligible effect on the aptasensor performance due to the high selectivity of the ATP aptamer to its target, showing promising potential in real sample analysis.

Water-Soluble Conjugated Polymer as a Fluorescent Probe for Monitoring Adenosine Triphosphate Level Fluctuation in Cell Membranes during Cell Apoptosis and in Vivo

Adenosine triphosphate (ATP) is used as the energy source in cells and plays crucial roles in various cellular events. The cellular membrane is the protective barrier for the cytoplasm of living cells and involved in many essential biological processes. Many fluorescent probes for ATP have been successfully developed, but few of these probes were appropriate for visualizing ATP level fluctuation in cell membranes during the apoptotic cell death process. Herein, we report the synthesis of a new water-soluble cationic polythiophene derivative that can be utilized as a fluorescent sensor for detecting ATP in cell membranes. Poly((3-((4-methylthiophen-3-yl)oxy)propyl)triphenylphosphonium chloride) (PMTPP) exhibits high sensitivity and good selectivity to ATP, and the detection limit is 27 nM. The polymer shows low toxicity to live cells and excellent photostability in cell membranes. PMTPP was practically utilized for real-time monitoring of ATP levels in the cell membrane through fluorescence microscopy. We have demonstrated that the ATP levels in cell membranes increased during the apoptotic cell death process. The probe was also capable of imaging ATP levels in living mice.

Simultaneous monitoring of the drug release and antitumor effect of a novel drug delivery system-MWCNTs/DOX/TC

Monitoring drug release and therapeutic efficacy is crucial for developing drug delivery systems. Our preliminary study demonstrated that, as compared with pristine multiwalled carbon nanotubes (MWCNTs), transactivator of transcription (TAT)-chitosan functionalized MWCNTs (MWCNTs-TC) were a more promising candidate for drug delivery in cancer therapy. In the present study, a MWCNTs/TC-based drug delivery system was developed for an anticancer drug, doxorubicin (DOX). The drug loading and in vitro release profiles, cellular uptake and cytotoxicity were assessed. More importantly, the in vivo drug release and antitumor effect of MWCNTs/DOX/TC were evaluated by noninvasive fluorescence and bioluminescence imaging. It was demonstrated that MWCNTs/DOX/TC can be efficiently taken up by BEL-7402 hepatoma cells. The release of DOX from MWCNTs/DOX/TC was faster under lower pH condition, which was beneficial for intrcellular drug release. The in vivo release process of DOX and antitumor effect in animal model were monitored simultaneously by noninvasive fluorescence and luminescence imaging, which demonstrated the application potential of MWCNTs/DOX/TC for cancer therapy.

Electrostatic gating in carbon nanotube aptasensors

Synthetic DNA aptamer receptors could boost the prospects of carbon nanotube (CNT)-based electronic biosensors if signal transduction can be understood and engineered. Here, we report CNT aptasensors for potassium ions that clearly demonstrate aptamer-induced electrostatic gating of electronic conduction. The CNT network devices were fabricated on flexible substrates via a facile solution processing route and non-covalently functionalised with potassium binding aptamers. Monotonic increases in CNT conduction were observed in response to increasing potassium ion concentration, with a level of detection as low as 10 picomolar. The signal was shown to arise from a specific aptamer-target interaction that stabilises a G-quadruplex structure, bringing high negative charge density near the CNT channel. Electrostatic gating is established via the specificity and the sign of the current response, and by observing its suppression when higher ionic strength decreases the Debye length at the CNT-water interface. Sensitivity towards potassium and selectivity against other ions is demonstrated in both resistive mode and real time transistor mode measurements. The effective device architecture presented, along with the identification of clear response signatures, should inform the development of new electronic biosensors using the growing library of aptamer receptors.

Small-Molecule Binding Aptamers: Selection Strategies, Characterization, and Applications

Aptamers are single-stranded, synthetic oligonucleotides that fold into 3-dimensional shapes capable of binding non-covalently with high affinity and specificity to a target molecule. They are generated via an in vitro process known as the Systematic Evolution of Ligands by EXponential enrichment, from which candidates are screened and characterized, and then used in various applications. These applications range from therapeutic uses to biosensors for target detection. Aptamers for small molecule targets such as toxins, antibiotics, molecular markers, drugs, and heavy metals will be the focus of this review. Their accurate detection is needed for the protection and wellbeing of humans and animals. However, the small molecular weights of these targets, including the drastic size difference between the target and the oligonucleotides, make it challenging to select, characterize, and apply aptamers for their detection. Thus, recent (since 2012) notable advances in small molecule aptamers, which have overcome some of these challenges, are presented here, while defining challenges that still exist are discussed.

Lysosomal ATP imaging in living cells by a water-soluble cationic polythiophene derivative

Lysosomes in astrocytes and microglia can release ATP as the signaling molecule for the cells through ca(2+)-dependent exocytosis in response to various stimuli. At present, fluorescent probes that can detect ATP in lysosomes have not been reported. In this work, we have developed a new water-soluble cationic polythiophene derivative that can be specifically localized in lysosomes and can be utilized as a fluorescent probe to sense ATP in cells. PEMTEI exhibits high selectivity and sensitivity to ATP at physiological pH values and the detection limit of ATP is as low as 10(-11)M. The probe has low cytotoxicity, good permeability and high photostability in living cells and has been applied successfully to real-time monitoring of the change in concentrations of ATP in lysosomes though fluorescence microscopy. We also demonstrated that lysosomes in Hela cells can release ATP through Ca(2+)-dependent exocytosis in response to drug stimuli.

Protein-templated cobaltous phosphate nanocomposites for the highly sensitive and selective detection of platelet-derived growth factor-BB

We synthesized novel Co3(PO4)2-based nanocomposites with 3D porous architectures via self-assembly; here, bovine serum albumin (BSA) and aptamer were used as organic phases to produce Co3(PO4)2@BSA and Co3(PO4)2@Apt nanocomposites, respectively. The formation mechanism of Co3(PO4)2-based nanocomposites was described based on characterizations of their physio-chemical performance, and the developed nanocomposites were applied as scaffold materials to construct a novel electrochemical aptasensor and detect platelet-derived growth factor-BB (PDGF-BB). The PDGF-BB targeting aptamer must be immobilized onto the Co3(PO4)2@BSA-modified electrode to detect PDGF-BB, whereas Co3(PO4)2@Apt-based aptasensor may be directly used to determine the target protein. Electrochemical impedance spectroscopy results showed that the developed Co3(PO4)2@BSA- and Co3(PO4)2@Apt-based aptasensors present highly sensitive detection ability toward PDGF-BB. Due to the special nanoflower structure, the Co3(PO4)2@BSA-based aptasensor features a detection limit of 3.7 pg mL(-1); while the limit of detection of the Co3(PO4)2@Apt-based aptasensor is 61.5 pg mL(-1), which is the possible bioactivity loss of the aptamer in Co3(PO4)2@Apt nanocomposite. The two detection limits obtained are still much lower than or comparable with those of previously reported aptasensors. The Co3(PO4)2@BSA- and Co3(PO4)2@Apt-based aptasensors showed high selectivity, stability, and applicability for detecting the desired protein. This finding indicates that the Co3(PO4)2-based nanocomposites could be used as an electrochemical biosensor for various detection procedures in the biomedical field.

Carbon nanotubes part I: preparation of a novel and versatile drug-delivery vehicle

INTRODUCTION

It is 23 years since carbon allotrope known as carbon nanotubes (CNT) was discovered by Iijima, who described them as "rolled graphite sheets inserted into each other". Since then, CNTs have been studied in nanoelectronic devices. However, CNTs also possess the versatility to act as drug- and gene-delivery vehicles.

AREAS COVERED

This review covers the synthesis, purification and functionalization of CNTs. Arc discharge, laser ablation and chemical vapor deposition are the principle synthesis methods. Non-covalent functionalization relies on attachment of biomolecules by coating the CNT with surfactants, synthetic polymers and biopolymers. Covalent functionalization often involves the initial introduction of carboxylic acids or amine groups, diazonium addition, 1,3-dipolar cycloaddition or reductive alkylation. The aim is to produce functional groups to attach the active cargo.

EXPERT OPINION

In this review, the feasibility of CNT being used as a drug-delivery vehicle is explored. The molecular composition of CNT is extremely hydrophobic and highly aggregation-prone. Therefore, most of the efforts towards drug delivery has centered on chemical functionalization, which is usually divided in two categories; non-covalent and covalent. The biomedical applications of CNT are growing apace, and new drug-delivery technologies play a major role in these efforts.

An aptameric graphene nanosensor for label-free detection of small-molecule biomarkers

This paper presents an aptameric graphene nanosensor for detection of small-molecule biomarkers. To address difficulties in direct detection of small molecules associated with their low molecular weight and electrical charge, we incorporate an aptamer-based competitive affinity assay in a graphene field effect transistor (FET), and demonstrate the utility of the nanosensor with dehydroepiandrosterone sulfate (DHEA-S), a small-molecule steroid hormone, as the target analyte. In the competitive affinity assay, DHEA-S specifically binds to aptamer molecules pre-hybridized to their complementary DNA anchor molecules immobilized on the graphene surface. This results in the competitive release of the strongly charged aptamer from the DNA anchor and hence a change in electrical properties of the graphene, which can be measured to achieve the detection of DHEA-S. We present experimental data on the label-free, specific and quantitative detection of DHEA-S at clinically appropriate concentrations with an estimated detection limit of 44.7 nM, and analyze the trend observed in the experiments using molecular binding kinetics theory. These results demonstrate the potential of our nanosensor in the detection of DHEA-S and other small molecules in biomedical applications.

A new and highly sensitive resonance Rayleigh scattering assay for lysozyme using aptamer-nanogold as a probe

Gold nanoparticles (GN), 10 nm in size, were modified by using lysozyme aptamer (Apt) to obtain a stable Apt–GN probe in pH 8.05 Tris/HCl buffer solutions containing 0.04 mol/L NaCl. Upon addition of lysozyme (LYS), it reacted with the Apt of the probe to form a very stable Apt–LYS complex and to release GNs, which aggregated to form large clusters with a resonance Rayleigh scattering (RRS) peak at 368 nm. The enhanced peak intensity, ΔI, was linear to the LYS concentration in the range 0.2–5.2 nmol/L, with a detection limit of 0.05 nmol/L. The influence of foreign substance was tested, and the results showed that this RRS method has high selectivity. This Apt–GN RRS method was applied to the analysis of LYS in a real sample, with satisfactory results.

Paper-based chemiresistor for detection of ultralow concentrations of protein

A new paper-based chemiresistor composed of a network of single-wall carbon nanotubes (SWCNTs) and anti-human immunoglobulin G (anti-HIgG) is reported herein. SWCNTs act as outstanding transducers because they provide high sensitivity in terms of resistance changes due to immunoreaction. As a result, the resistance-based biosensor reaches concentration detection as low as picomolar. The resulting paper-based biosensor is sensitive, selective and employs low-cost substrate and simple manufacturing stages. Since chemiresistors require low-power equipment and are able to detect low concentrations with inexpensive materials, the present approach may pave the way for the development of resistive biosensors at very low-cost with high performances.

Label-free aptasensor for thrombin determination based on the nanostructured phenazine mediator

New aptasensors based on DNA aptamer and polycarboxylated thiacalix[4]arenes in cone, 1,3-alternate and partial cone configurations bearing Neutral Red (NR) at substituents at the lower rim have been developed and applied for thrombin detection. The assembly of the biorecognition layer was optimized by AFM and EIS study to reach the maximal coverage and regular composition of the surface layer. The interaction of the NR groups with thrombin suppressed the electron hopping between oxidized and reduced mediator groups. This regularly decreased the NR peak current and increased the resistance of the charge transfer. The aptasensor makes it possible to detect from 1 nM to 1 μM of thrombin with the detection limit of 0.05-0.5 nM. No effect of the 1000 excess of bovine serum albumin on the signal was observed. The influence of thiacalix[4]arene configuration on the sensitivity of aptasensor signal toward thrombin is discussed.

Au(III)-assisted core-shell iron oxide@poly(o-phenylenediamine) nanostructures for ultrasensitive electrochemical aptasensors based on DNase I-catalyzed target recycling

A redox-active Au(III)-assisted core-shell iron oxide@poly(o-phenylenediamine) nanostructure was designed as a sensing platform for ultrasensitive electrochemical detection of small molecules (ATP, used as a model here) by coupling with DNase I-catalyzed target recycling.

Graphene and other nanomaterial-based electrochemical aptasensors

Electrochemical aptasensors, which are based on the specificity of aptamer-target recognition, with electrochemical transduction for analytical purposes have received particular attention due to their high sensitivity and selectivity, simple instrumentation, as well as low production cost. Aptamers are functional nucleic acids with specific and high affinity to their targets, similar to antibodies. However, they are completely selected in vitro in contrast to antibodies. Due to their stability, easy chemical modifications and proneness to nanostructured device construction, aptamer-based sensors have been incorporated in a variety of applications including electrochemical sensing devices. In recent years, the performance of aptasensors has been augmented by incorporating novel nanomaterials in the preparation of better electrochemical sensors. In this review, we summarize the recent trends in the use of nanomaterials for developing electrochemical aptasensors.