Design, synthesis, and characterization of nucleic-acid-functionalized gold surfaces for biomarker detection

Gold nanourchin on multiple-point dielectrode for glucose biosensing by current-potential measurement

Gestational diabetes (GD) is a condition characterized by elevated blood sugar levels during pregnancy. GD poses various health risks, such as serious birth injuries, the need for cesarean delivery, and the necessity of newborn care. Monitoring glucose levels is essential for ensuring safe delivery and reducing the risks to both the mother and fetus. Various sensors are readily available for monitoring glucose levels, and researchers are continually working to develop highly sensitive glucose sensors. This research aimed to develop a gold nanourchin (AuNU)-hybrid biosensor for quantifying glucose on a multi-point electrode sensor. Glucose oxidase (GOx) was attached to the AuNU and seeded on the sensing surface using an amine linker. The current-potential (1-2 V at 0.1 V sweep) was recorded for the GOx-glucose interaction, with a limit of detection of 560 μM and a regression coefficient (R2) of 0.9743 [y = 0.9106x - 0.9953] on the linear curve. The sensitivity was estimated to be 3.5 mAcm-2M-1. Furthermore, control experiments with galactose, sucrose, and fructose did not yield an increase in current-potential, confirming specific glucose detection. This experiment helps in monitoring glucose levels to manage conditions associated with GD.

Development and characterization of functionalized glyco thiolate capped gold nanoparticles for biological applications

Glyco-gold nanoparticles (AuNPs) in aqueous dispersions were prepared by two approaches, namely direct reduction and ligand substitution methods. In the direct method, potassium salts of glyco thiols, with the general formula (C6H11O6)NH(CH2) n CH2SK (where L1, n = 1; L2, n = 2; L3, n = 3, L4, n = 4; L5, n = 5), were used as reducing and capping agents to give the glyco thiolate capped gold nanoparticles (AuNPs G1-G5); meanwhile in the ligand exchange experiments, L1-L5 and their acetylated forms (L6-L8) replaced citrate ions in citrate-capped gold nanoparticles to give additional AuNPs G6-G11. UV-visible spectroscopy, surface charge (ζ-potential,) measurements and transmission electron microscopy (TEM) were used for physical and chemical characterization of all the resultant AuNPs. The ζ-potential studies of AuNPs prepared through the direct method revealed that the surface charge is dependent on the length of the alkyl unit of (C6H11O6)NH(CH2) n CH2S- ligands. TEM images of the acetylated and non-acetylated glyco thiolate capped gold nanoparticles (AuNPs G6-G11) prepared via the ligand exchange method indicate that the size and shape of the gold nanoparticles remained the same as those of the citrate-capped gold nanoparticles used to prepare them. Selected AuNPs were tested on peripheral blood mononuclear cells (PBMCs) and the A549 cancer cell line to investigate their respective toxicity and cytotoxicity profiles. All AuNPs showed indiscriminate activity against both PBMCs and A4549 cells, although the gold nanoparticles having an acetylated glyco moiety with an amino propyl thiol linker as the ligand (G10) prepared via the citrate exchange method had better selectivity (PBMCs >59 mg mL-1 and for A549 ∼7 μg mL-1).

Overview of Optical and Electrochemical Alkaline Phosphatase (ALP) Biosensors: Recent Approaches in Cells Culture Techniques

Alkaline phosphatase (ALP), which catalyzes the dephosphorylation process of proteins, nucleic acids, and small molecules, can be found in a variety of tissues (intestine, liver, bone, kidney, and placenta) of almost all living organisms. This enzyme has been extensively used as a biomarker in enzyme immunoassays and molecular biology. ALP is also one of the most commonly assayed enzymes in routine clinical practice. Due to its close relation to a variety of pathological processes, ALP's abnormal level is an important diagnostic biomarker of many human diseases, such as liver dysfunction, bone diseases, kidney acute injury, and cancer. Therefore, the development of convenient and reliable assay methods for monitoring ALP activity/level is extremely important and valuable, not only for clinical diagnoses but also in the area of biomedical research. This paper comprehensively reviews the strategies of optical and electrochemical detection of ALP and discusses the electrochemical techniques that have been addressed to make them suitable for ALP analysis in cell culture.

Development of a Label-Free Electrochemical Aptasensor for the Detection of Tau381 and its Preliminary Application in AD and Non-AD Patients' Sera

The electrochemical aptamer sensor has been designed for detecting tau381, a critical biomarker of Alzheimer's disease in human serum. The aptasensor is obtained by immobilizing the aptamer on a carboxyl graphene/thionin/gold nanoparticle modified glassy-carbon electrode. As a probe and bridge molecule, thionin connected carboxyl graphene and gold nanoparticles, and gave the electrical signal. Under optimal conditions, the increment of differential pulse voltammetry signal increased linearly with the logarithm of tau381 concentration in the range from 1.0 pM to 100 pM, and limit of detection was 0.70 pM. The aptasensor reliability was evaluated by determining its selectivity, reproducibility, stability, detection limit, and recovery. Performance analysis of the tau381 aptasensor in 10 patients' serum samples showed that the aptasensor could screen patients with and without Alzheimer's disease. The proposed aptasensor has potential for use in clinically diagnosing Alzheimer's disease in the early stage.

Naphthalimide-Based DNA-Coupled Hybrid Assembly for Sensing Dipicolinic Acid: A Biomarker for Bacillus anthracis Spores

We have designed and synthesized a novel, water-soluble naphthalimide-histidine receptor (1) with excellent fluorescent properties. Functioning of the synthesized receptor was performed through developing their DNA-receptor hybrid assembly (DRHA), which has shown significant changes in the emission profile upon interactions with dipicolinic acid (DPA), a biomarker for Bacillus anthracis spores. DRHA showed fluorescence enhancement upon binding with DPA with the characteristic of internal charge transfer. It is notable that this assembly exhibited a significant limit of detection (12 nM) toward DPA. The mechanism of sensing was fully defined using ethidium bromide (EtBr) interaction studies as well as Fourier transform infrared spectroscopic analysis, which describes the binding mode of DRHA with DPA. This assembly selectively interacts with DPA over other anions, common cellular cations, and aromatic acids in aqueous media.

Rapid, Surfactant-Free, and Quantitative Functionalization of Gold Nanoparticles with Thiolated DNA under Physiological pH and Its Application in Molecular Beacon-Based Biosensor

The controlled attachment of thiolated DNA to gold nanoparticles (AuNPs) dictates many applications. This is typically achieved by either "aging-salting" processes or low-pH method, where either Na⁺ or H⁺ is used to minimize charge repulsion and facilitate attachment of thiolated DNA onto AuNPs. However, the "aging-salting" process takes a long time, and is prone to aggregation when used with larger AuNPs. Surfactants are needed to precoat and thereby enhance the stability of AuNPs. The low-pH method can disrupt the structural integrity of DNAs. We report here an oligoethylene glycol (OEG) spacer-assisted method that enables quantitative and instantaneous attachment at physiological pH without the need for surfactants. The method is based on our finding that an uncharged OEG spacer as short as six EG units can effectively shield against repulsion between AuNPs and DNAs, substantially enhancing both the adsorption kinetics and thermodynamics of thiolated DNAs. We applied this to thiolated DNAs of various lengths and thiol modification positions and to large AuNPs. Importantly, our method also allows for the direct immobilization of thiolated molecular beacons (MB), and avoids particle aggregation due to intermolecular hydrogen bonding. The prepared MB-AuNPs were successfully used for the fluorescent detection of target DNA at nanomolar concentrations. The OEG spacer appears to offer a highly effective parameter for tuning DNA adsorption kinetics and thermodynamics besides pH and salt, providing a novel means for highly controllable and versatile functionalization of AuNPs.

Tuning Toehold Length and Temperature to Achieve Rapid, Colorimetric Detection of DNA from the Disassembly of DNA-Gold Nanoparticle Aggregates

Gold nanoparticles have been widely utilized to achieve colorimetric detection for various diagnostic applications. One of the most frequently used methods for DNA detection involves the aggregation of DNA-modified gold nanoparticles driven by target DNA hybridization. This process, however, is intrinsically slow, limiting its use in rapid diagnostics. Here we take advantage of the reverse process: the disassembly of preformed aggregates triggered by the addition of target DNA via a strand displacement mechanism. A systematic study of the dependence of the disassembly rate on temperature, with and without toeholds, has delivered a system that produces an extremely rapid colorimetric response. Furthermore, using an optimal toehold length of 5 nucleotides, target triggered disassembly is rapid over a wide range of ambient temperatures. Using this overhang system, simple visualization of low picomole amounts of target DNA is possible within 10 min at room temperature.

Highly sensitive visual detection of catalase based on the accelerating decomposition of H2O2 using Au nanorods as a sensor

Novel colourimetric strategy was developed to the selective and rapid visual detection of catalase by gold nanorod decelerating etching.

Poly(N-isopropylacrylamide) coated gold nanoparticles as colourimetric temperature and salt sensors

Thermoresponsive PNIPAM coated AuNPs having dual stabilisation by the polymer chains and the charges, has been utilised for the development of temperature and salt sensors.

[The Increase of Oligonucleotides--Gold Nanoparticles Conjugates Stability]

For today the reagents based on oligonucleotides--gold nanoparticles conjugates and used for specific nucleic acids detection are actively being developed. Such molecular structures are stabilized through the bonds between thiol group in oligonucleotides and gold atoms in nanoparticle. The durability of oligonucleotides--gold nanoparticles binding affects directly on the stability of conjugates and on the possibility of further manipulations. In this paper, a method for the strengthening of oligonucleotides attachment on the gold nano-particles surface by means of anchor groups with dithiolane residues is proposed. A comparative study of the anchors molecular structure influence on the conjugates stability at conditions that typical for oligonucleotide probes was carried out.

A multiplex fluorophore molecular beacon: detection of the target sequence using large Stokes shift and multiple emission signal properties

We have developed a multiplex fluorophore molecular beacon (mfMB) with fluorophores located at its end to produce unique FRET (Fluorescence Resonance Energy Transfer). It exhibited diverse fluorescence properties depending on the mixing pattern, such as large Stokes shift emission and multiple colors.

Amphiphilic polyether-based block copolymers as crosslinkable ligands for Au-nanoparticles

We report on the synthesis of thiol-terminated, polyether-based block copolymers featuring a crosslinkable block and their use as ligands for Au-nanoparticles in organic solvents.

Label-free chemiluminescent aptasensor for platelet-derived growth factor detection based on exonuclease-assisted cascade autocatalytic recycling amplification

Here an exonuclease III (Exo III)-assisted cascade autocatalytic recycling amplification (Exo-CARA) strategy is proposed for label-free chemiluminescent (CL) detection of platelet-derived growth factor BB (PDGF-BB) by taking advantage of both recognition property of aptamer and cleavage function of Exo III. Functionally, this system consists of a duplex DNA (aptamer-blocker hybrid), two kinds of hairpin structures (MB1 and MB2), and Exo III. Upon recognizing and binding with PDGF-BB, aptamer folds into a close configuration, which initiates the proposed Exo-CARA reaction (Recyclings I→II→III→II). Finally, numerous "caged" G-quadruplex sequences on DNAzyme1 and DNAzyme2 release that intercalate hemin to catalyze the oxidation of luminol by H2O2 to generate an amplified CL signal, achieving excellent specificity and high sensitivity with a detection limit of 6.8×10(-13) M PDGF-BB. The proposed strategy has the advantages of simple design, isothermal conditions, homogeneous reaction without separation and washing steps, effective-cost without the need of labeling, and high amplification efficiency, which might be a universal and promising protocol for the detection of a variety of biomolecules whose aptamers undergo similar conformational changes.

Detection of potential microbial antigens by immuno-PCR (PCR-amplified immunoassay)

Immuno-PCR (PCR-amplified immunoassay; I-PCR) is a novel ultrasensitive method combining the versatility of ELISA with the sensitivity of nucleic acid amplification of PCR. The enormous exponential amplification power of PCR in an I-PCR assay leads to at least a 10(2)-10(4)-fold increase in sensitivity compared with an analogous ELISA. I-PCR has been used to detect many biological molecules such as proto-oncogenes, toxins, cytokines, hormones, and biomarkers for autoimmune and Alzheimer's diseases, as well as microbial antigens and antibodies, and it can be adapted as a novel diagnostic tool for various infectious and non-infectious diseases. Quantitative real-time I-PCR has the potential to become the most analytically sensitive method for the detection of proteins. The sensitivity and specificity of a real-time I-PCR assay can be enhanced further with the use of magnetic beads and nanoparticles. This review is primarily focused on the detection of potential viral, bacterial and parasitic antigens by I-PCR assay, thus enabling their application for immunological research and for early diagnosis of infectious diseases.

Enhancing on-surface mismatch discrimination capability of PNA probes by AuNP modification of gold(111) surface

Unambiguous identification of single base mismatches in nucleic acid sequences is of great importance in nucleic acid detection assays. However, ambiguities are often encountered with, and therefore, a strategy for attaining substantially large enhancement of mismatch discrimination has been worked upon in this study. Short single-stranded peptide nucleic acid (PNA) and deoxyribonucleic acid (DNA) sensor probes that are immobilized onto gold nanoparticle (AuNP) modified Au(111) surface have been applied for target DNA detection. It will be shown that while both PNA and the analogous DNA probes exhibit generally better target detection abilities on the AuNP-modified Au(111) surface (elicited from fluorescence-based measurement of on-surface Tm values), compared to the bare Au(111) surface, PNA supersedes DNA, for all sizes of AuNPs (10, 50, and 90 nm) applied, with the difference being quite drastic in the case of the smallest 10 nm AuNP. It is found that while the AuNP curvature plays a pivotal role in target detection abilities of the PNA probes, the changes in the surface roughness caused by AuNP treatment do not exert any significant influence. This study also presents a means for preparing PNA-AuNP hybrids without altering PNA functionality and without AuNP aggregation by working with the surface-affixed AuNPs.

Aptamer-linked biosensor for thrombin based on AuNPs/thionine-graphene nanocomposite

A novel protocol of electrochemical thrombin aptasensor based on a goldnanoparticles/thionine-graphene (AuNPs/Th-G) nanocomposite modified glassy carbon electrode was presented. Graphene was non-covalently functionalized by thionine viaπ-stacking interaction, and then AuNPs were electrodeposited onto the Th-G surface. The morphology and conductivity of the AuNPs/Th-G nanocomposite were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The ethanethiol-substituted oligonucleotide probe was immobilized onto the surface of the AuNPs/Th-G nanocomposite to prepare an electrochemical biosensing platform. The aptamer-thrombin reaction was monitored by differential pulse voltammetry. Under optimum conditions, the proposed biosensor exhibits high sensitivity and a low detection limit for thrombin determination. The thrombin could be quantified in a wide range of 0.5 to 40 nM and with a low detection limit of 0.093 nM (S/N = 3).

A label-free electrochemiluminescence aptasensor for thrombin based on novel assembly strategy of oligonucleotide and luminol functionalized gold nanoparticles

In the work, a label-free electrochemiluminescence (ECL) aptasensor for the sensitive and selective detection of thrombin was constructed based on target-induced direct ECL signal change by virtue of a novel assembly strategy of oligonucleotide and luminol functionalized gold nanoparticles (luminol-AuNPs). It is the first label-free ECL biosensor based on luminol and its analogs functionalized AuNPs. Streptavidin AuNPs coated with biotinylated DNA capture probe 1 (AuNPs-probe 1) were firstly assembled onto an gold electrode through 1,3-propanedithiol. Then luminol-AuNPs co-loaded with thiolated DNA capture probe 2 and thiolated thrombin binding aptamer (TBA) (luminol-AuNPs-probe 2/TBA) were assembled onto AuNPs-probe 1 modified electrode through the hybridization between capture probes 1 and 2. The luminol-AuNPs-probe 2/TBA acted as both molecule recognition probe and sensing interface. An Au/AuNPs/ds-DNA/luminol-AuNPs/TBA multilayer architecture was obtained. In the presence of target thrombin, TBA on the luminol-AuNPs could capture the thrombin onto the electrode surface, which produced a barrier for electro-transfer and influenced the electro-oxidation reaction of luminol, leading to a decrease in ECL intensity. The change of ECL intensity indirectly reflected the concentration of thrombin. Thus, the approach showed a high sensitivity and a wider linearity for the detection of thrombin in the range of 0.005-50nM with a detection limit of 1.7pM. This work reveals that luminol-AuNPs are ideal platform for label-free ECL bioassays.