An aptamer based on-plate microarray for high-throughput insulin detection by MALDI-TOF MS

Insulin detection in diabetes mellitus: challenges and new prospects

Tremendous progress has been made towards achieving tight glycaemic control in individuals with diabetes mellitus through the use of frequent or continuous glucose measurements. However, in patients who require insulin, accurate dosing must consider multiple factors that affect insulin sensitivity and modulate insulin bolus needs. Accordingly, an urgent need exists for frequent and real-time insulin measurements to closely track the dynamic blood concentration of insulin during insulin therapy and guide optimal insulin dosing. Nevertheless, traditional centralized insulin testing cannot offer timely measurements, which are essential to achieving this goal. This Perspective discusses the advances and challenges in moving insulin assays from traditional laboratory-based assays to frequent and continuous measurements in decentralized (point-of-care and home) settings. Technologies that hold promise for insulin testing using disposable test strips, mobile systems and wearable real-time insulin-sensing devices are discussed. We also consider future prospects for continuous insulin monitoring and for fully integrated multisensor-guided closed-loop artificial pancreas systems.

Construction of a Carcinoembryonic Antigen Surface-Enhanced Raman Spectroscopy (SERS) Aptamer Sensor Based on the Silver Nanorod Array Chip

Carcinoembryonic antigen (CEA) is a cancer-related tumor marker, which is commonly used for preventive screening, auxiliary diagnosis, and recurrence monitoring. Therefore, it is of great significance to develop a new CEA detection method. In this paper, we developed an SERS aptasensor for CEA based on silver nanorod array chip, thiol aptamer, and 4-mercaptophenylboronic acid (4-MPBA). The silver nanorod array chip modified by CEA thiol aptamer (aptamer-SH) was used as SERS capture substrates. Ag@4-MPBA was used as a surface-enhanced Raman spectroscopy (SERS) tag. This proposed SERS aptasensor could detect CEA down to 0.447 pg·mL^-1 with a wide linear range from 1 pg·mL^-1 to 100 ng·mL^-1 (R² = 0.9907). The recovery of the standard addition test for CEA in serum was between 97.25% and 102.67%, and the RSD ≤ 2.52% (n = 3). The sensor has the advantages of good specificity, high sensitivity, and a wide linear range. It provides a new method for the detection of CEA in serum.

MOF-Derived Cu-BTC Nanowire-Embedded 2D Leaf-like Structured ZIF Composite-Based Aptamer Sensors for Real-Time In Vivo Insulin Monitoring

Insulin, which is a hormone produced by the β-cells of the pancreas, regulates the glucose levels in the blood and can transport glucose into cells to produce glycogen or triglycerides. Insulin deficiency can lead to hyperglycemia and diabetes. Therefore, insulin detection is critical in clinical diagnosis. In this study, disposable Au electrodes were modified with copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC)/leaf-like zeolitic imidazolate framework (ZIF-L) for insulin detection. The aptamers are easily immobilized on the Cu-BTC/ZIF-L composite by physical adsorption and facilitated the specific interaction between aptamers and insulin. The Cu-BTC/ZIF-L composite-based aptasensor presented a wide linear insulin detection range (0.1 pM to 5 μM) and a low limit of detection of 0.027 pM. In addition, the aptasensor displayed high specificity, good reproducibility and stability, and favorable practicability in human serum samples. For the in vivo tests, Cu-BTC/ZIF-L composite-modified electrodes were implanted in non-diabetic and diabetic mice, and insulin was quantified using electrochemical and enzyme-linked immunosorbent assay methods.

Capture of Phenylalanine and Phenylalanine-Terminated Peptides Using a Supramolecular Macrocycle for Surface-Enhanced Raman Scattering Detection

The cucurbit[n]uril (CB[n]) family of macrocycles are known to bind a variety of small molecules with high affinity. These motifs thus have promise in an ever-growing list of trace detection methods. Surface-enhanced Raman scattering (SERS) detection schemes employing CB[n] motifs exhibit increased sensitivity due to selective concentration of the analyte at the nanoparticle surface, coupled with the ability of CB[n] to facilitate the formation of well-defined electromagnetic hot spots. Herein, we report a CB[7] SERS assay for quantification of phenylalanine (Phe) and further demonstrate its utility for detecting peptides with an N-terminal Phe. The CB[7]-guest interaction improves the sensitivity 5-25-fold over direct detection of Phe using citrate-capped silver nanoparticle aggregates, enabling use of a portable Raman system. We further illustrate detection of insulin via binding of CB[7] to the N-terminal Phe residue on its B-chain, suggesting a general strategy for detecting Phe-terminated peptides of clinically relevant biomolecules.

A rational route to hybrid aptamer-molecularly imprinted magnetic nanoprobe for recognition of protein biomarkers in human serum

Although antibody has played a great role in highly specific recognition of protein biomarkers, it faces poor stability, reproducibility, high-cost and time-consuming preparation, etc. Here, aptamer and molecularly imprinted polymers (MIPs), both as promising substitutes of antibody, were integrated onto magnetic nanoparticles by Au-S bonds and SiO₂ as imprinted layer for preparing a new nanoprobe. Highly specific and sensitive recognition of different protein biomarkers, such as insulin for diabetes and alpha-fetoprotein (AFP) for hepatic carcinoma, were achieved respectively by the system of combining hybrid aptamer-molecularly imprinted magnetic nanoprobe and mass spectrometry. With the double affinities offered by aptamer-MIPs, insulin can be detected at 0.5 ng mL^-1 in human serum dilution, the equlibrium dissociation constant between nanoprobe and insulin is measured as 23.61 ± 2.27 μM. Likewise, AFP can be sufficiently detected in human saliva dilution from 1000 ng mL^-1 to 20 ng mL^-1, and two patients with hepatic carcinoma are discriminated from healthy person due to the abnormally high expression of AFP in serum.

Aptamer-gold nanoparticle doped covalent organic framework followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for selective enrichment and detection of human insulin

In this work, we introduced an aptamer modified Au nanoparticles doped covalent organic frameworks composite (IBAs-AuNPs/COF) to improve the property of selective enrichment of insulin from serum samples. The Au nanoparticles were immobilized on imine-based COF by in-situ reduction reaction via mussel inspired polydopamine coating, and then sulfhydryl-containing aptamers were bonded to the surface of AuNPs through an Au-S linkage. Due to the excellent adsorption property of COF and specific recognition between insulin and IBAs, the IBAs-AuNPs/COF composites show selective and satisfactory extraction property to insulin in serum samples. Excellent specifity was obtained for insulin in the presence of 50-fold interfering substances including human immunoglobulin, lysozyme and biotin. The concentrations of insulin in the range of 1.0 to 50.0 μg L^-1 show good linear relationship (R² = 0.9917) with limit of detection and limit of quantitation of 0.28 μg L^-1 and 0.93 μg L^-1, respectively. Then, the IBAs-AuNPs/COF composites were applied to enrich insulin in serum samples followed by analysis with matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS). After the recovery experiment, the developed method shows good recoveries in range of 91.6%-112.4% with low RSD value (2.4%-9.4%, n = 3) for diabetic and healthy serum samples. The developed IBAs-AuNPs/COF composites propose a new perspective for selective and efficient enrichment of biomarkers in serum samples by functionalized COF.

Aptamer-functionalized magnetic metal organic framework as nanoprobe for biomarkers in human serum

Human serum is a huge bioinformatics database of human physiological and pathological state, many proteins/peptides among which can serve as biomarkers for monitoring human's health condition, thereby being worth exploring. The simple and fast capture of biomarkers from human serum is the first key step to realize their accurate detection. In this work, we developed the aptamer functionalized magnetic metal organic framework nanoprobe, and furtherly combined with mass spectrometry technology to establish an efficient method of identifying biomarkers. Taking insulin as example of biomarker in human serum, we developed sulfhydryl human insulin aptamer functionalized magnetic metal organic framework (denoted as Mag MOF@Au@HIA) through the post-synthetic modification of MIL-101(Cr)-NH₂ for testing the applicability of the established method. Depending on the strong magnetic responsiveness and high specific area as well as high-loaded human insulin aptamers, the limit of detection of insulin was down to 1 ng/mL and 2 ng/mL in the standard insulin solution and human serum, respectively. Moreover, a good linear relationship (R² = 0.998) was obtained by using standard insulin solution with concentration range from 100 ng/mL to 5 ng/mL, based on which the capture recovery of insulin with Mag MOF@Au@HIA from human serum was demonstrated to be excellent. All of the results indicate that the aptamer-functionalized magnetic metal organic framework is a promising nanoprobe for biomarkers capture in human serum.

Towards Point-of-Care Insulin Detection

Good glucose management through an insulin dose regime based on the metabolism of glucose helps millions of people worldwide manage their diabetes. Since Banting and Best extracted insulin, glucose management has improved due to the introduction of insulin analogues that act from 30 minutes to 28 days, improved insulin dose regimes, and portable glucose meters, with a current focus on alternative sampling sites that are less invasive. However, a piece of the puzzle is still missing-the ability to measure insulin directly in a Point-of-Care device. The ability to measure both glucose and insulin concurrently will enable better glucose control by providing an improved estimate for insulin sensitivity, minimizing variability in control, and maximizing safety from hypoglycaemia. However, direct detection of free insulin has provided a challenge due to the size of the molecule, the low concentration of insulin in blood, and the selectivity against interferants in blood. This review summarizes current insulin detection methods from immunoassays to analytical chemistry, and sensors. We also discuss the challenges and potential of each of the methods towards Point-of-Care insulin detection.

Triple-helix molecular switch-based aptasensors and DNA sensors

Utilization of traditional analytical techniques is limited because they are generally time-consuming and require high consumption of reagents, complicated sample preparation and expensive equipment. Therefore, it is of great interest to achieve sensitive, rapid and simple detection methods. It is believed that nucleic acids assays, especially aptamers, are very important in modern life sciences for target detection and biological analysis. Aptamers and DNA-based sensors have been widely used for the design of various sensors owing to their unique features. In recent years, triple-helix molecular switch (THMS)-based aptasensors and DNA sensors have been broadly utilized for the detection and analysis of different targets. The THMS relies on the formation of DNA triplex via Watson-Crick and Hoogsteen base pairings under optimal conditions. This review focuses on recent progresses in the development and applications of electrochemical, colorimetric, fluorescence and SERS aptasensors and DNA sensors, which are based on THMS. Also, the advantages and drawbacks of these methods are discussed.

Sensitive colorimetric assay using insulin G-quadruplex aptamer arrays on DNA nanotubes coupled with magnetic nanoparticles

Described here is a methodology for fabrication of a sensitive colorimetric nanoassay for measurement of insulin using G-quadruplex aptamer arrays on DNA nanotubes (DNTs) coupled with magnetic nanoparticles. The spectroscopic findings (e.g. visible spectra, velocity assay and limit of detection determination) indicated a highly sensitive performance of this new nanoassay in comparison to those results obtained from the insulin assay with non-arrayed aptamers. The clinical performance statistics (i.e. paired sample t-test, Bland-Altman plot and scatter diagram) from the newly developed assay and the enzyme-linked immunosorbent assay suggested its reliable precision and its acceptable repeatability for measurement of insulin in human sera. This is, to our knowledge, the first study for the application of magnetic nanoparticle-coupled DNTs for carrying G-quadruplex aptamers for detection of biomolecules (such as insulin) in human serum.

Surface-Enhanced Raman Spectroscopy-Based Label-Free Insulin Detection at Physiological Concentrations for Analysis of Islet Performance

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

Dual-responsive electrochemical immunosensor for detection of insulin based on dual-functional zinc silicate spheres-palladium nanoparticles

In this study, described an electrochemical immunoassay for insulin that is based on the use of zinc silicate spheres loaded with palladium nanoparticles (Zn₂SiO₄-PdNPs) that act as dual-function labels. The Zn₂SiO₄-PdNPs display high electrocatalytic activity towards the reduction of H₂O₂ and high sensitivity in chronoamperometry. The Zn₂SiO₄-PdNPs decrease the electron transfer rate between the electrolyte and the surface of the electrode, which can increase the changed current and enhance the sensitivity of the immunosensor as detected by square wave voltammetry (SWV). Electrodeposited gold is used as the matrix material. The icosahedral gold nanocrystals are coated with the primary antibodies formed a 3D mode to against abundant of insulin. Under optimal conditions, the assay has a linear response in the 0.1pgmL^-1 to 50ngmL^-1 insulin concentration range, and the limit of detection of the SWV and CA methods are 0.25 fg mL^-1 and 80 fg mL^-1, respectively. Moreover, the immunosensor holds an outstanding analytical performance for the insulin detection and has promising potential in clinical diagnosis.

Immune-enrichment of insulin in bio-fluids on gold-nanoparticle decorated target plate and in situ detection by MALDI MS

BACKGROUND

Detection of low-abundance biomarkers using mass spectrometry (MS) is often hampered by non-target molecules in biological fluids. In addition, current procedures for sample preparation increase sample consumption and limit analysis throughput. Here, a simple strategy is proposed to construct an antibody-modified target plate for high-sensitivity MS detection of target markers such as insulin, in biological fluids.

METHODS

The target plate was first modified with gold nanoparticle, and then functionalized with corresponding antibody through chemical conjugation. Clinical specimens were incubated onto these antibody-functionalized target plates, and then subjected to matrix assisted laser desorption ionization mass spectrometry analysis.

RESULTS

Insulin in samples was enriched specifically on this functional plate. The detection just required low-volume samples (lower than 5 µL) and simplified handling process (within 40 min). This method exhibited high sensitivity (limit of detection in standard samples, 0.8 nM) and good linear correlation of MS intensity with insulin concentration (R² = 0.994). More importantly, insulin present in real biological fluids such as human serum and cell lysate could be detected directly by using this functional target plate without additional sample preparations.

CONCLUSIONS

Our method is easy to manipulate, cost-effective, and with a potential to be applied in the field of clinical biomarker detection.

Mesoporous materials modified by aptamers and hydrophobic groups assist ultra-sensitive insulin detection in serum

A novel mesoporous material modified with both insulin-binding-aptamers and hydrophobic methyl groups is synthesized. With rationally designed pore structures and surface chemistry, this material is applied in sample pre-treatment for ELISA, and enables the quantification (0.25-5 pg ml(-1)) of insulin in serum, 30-fold enhancement of the limit-of-detection compared to the commercial ELISA kit.

Molecular recognition between insulin and dextran encapsulated gold nanoparticles

Insulin is a peptide hormone that can regulate the metabolism of carbohydrates and lipids. This hormone is closely related to glucose-uptake in cells and can control blood glucose levels. Dextran is a polysaccharide composed of glucose units. In this study, we discovered that dextran-encapsulated gold nanoparticles (AuNPs@Dextran) and nanoclusters (AuNCs@Dextran) can be used to recognize insulin. The dissociation constant of insulin toward AuNPs@Dextran was estimated to be ∼5.3 × 10^-6  M. The binding site on insulin toward the dextran on the nanoprobes was explored as well. It was found that the sequence of numbers 1-22 on the insulin B chain can interact with the dextran encapsulated nanoprobes. Additionally, we also demonstrated that the dextran-encapsulated nanoprobes could be used as concentration probes to selectively enrich trace amounts of insulin (∼1 pM) from serum samples. Copyright © 2016 John Wiley & Sons, Ltd.

A methodological approach based on gold-nanoparticles followed by matrix assisted laser desorption ionization time of flight mass spectrometry for the analysis of urine profiling of knee osteoarthritis

The aim of this work is to develop a nanoparticle-based methodology to find out biomarkers of diagnostic for knee osteoarthritis, KOA, through the use of matrix assisted laser desorption ionization time-of-flight-based mass spectrometry profiling. Urine samples used for this study were obtained from KOA patients (42 patients), patients with prosthesis (58 patients), and controls (36 individuals) with no history of joint disease. Gold-nano particle MALDI-based urine profiling was optimized and then applied over the 136 individuals. Jaccard index and 10 different classifiers over MALDI MS datasets were used to find out potential biomarkers. Then, the specificity and sensitivity of the method were evaluated. The presence of ten m/z signals as potential biomarkers in the healthy versus non-healthy approach suggests that patients (KOA and prosthesis) are differentiable from the healthy volunteers through profiling. The automatic diagnostic study confirmed these preliminary conclusions. The sensitivity and the specificity for the urine profiling criteria here reported, achieved by the C4.5 classifier, is 97% and 69% respectively. Thus, it is confirmed the utility of the method proposed in this work as an additional fast, non-expensive and robust test for KOA diagnostic. When the proposed method is compared with those used in common practice it is found that sensitivity is the highest, thus with a low false negative rate for diagnostic KOA patients in the population studied. Specificity is lower but in the range accepted for diagnostic objectives.

Designed synthesis of aptamer-immobilized magnetic mesoporous silica/Au nanocomposites for highly selective enrichment and detection of insulin

We designed and synthesized aptamer-immobilized magnetic mesoporous silica/Au nanocomposites (MMANs) for highly selective detection of unlabeled insulin in complex biological media using MALDI-TOF MS. The aptamer was easily anchored onto the gold nanoparticles in the mesochannels of MMANs with high capacity for highly efficient and specific enrichment of insulin. With the benefit from the size-exclusion effect of the mesoporous silica shell with a narrow pore size distribution (∼2.9 nm), insulin could be selectively detected despite interference from seven untargeted proteins with different size dimensions. This method exhibited an excellent response for insulin in the range 2-1000 ng mL(-1). Moreover, good recoveries in the detection of insulin in 20-fold diluted human serum were achieved. We anticipate that this novel method could be extended to other biomarkers of interest and potentially applied in disease diagnostics.

Microarrays for identifying binding sites and probing structure of RNAs

Oligonucleotide microarrays are widely used in various biological studies. In this review, application of oligonucleotide microarrays for identifying binding sites and probing structure of RNAs is described. Deep sequencing allows fast determination of DNA and RNA sequence. High-throughput methods for determination of secondary structures of RNAs have also been developed. Those methods, however, do not reveal binding sites for oligonucleotides. In contrast, microarrays directly determine binding sites while also providing structural insights. Microarray mapping can be used over a wide range of experimental conditions, including temperature, pH, various cations at different concentrations and the presence of other molecules. Moreover, it is possible to make universal microarrays suitable for investigations of many different RNAs, and readout of results is rapid. Thus, microarrays are used to provide insight into oligonucleotide sequences potentially able to interfere with biological function. Better understanding of structure-function relationships of RNA can be facilitated by using microarrays to find RNA regions capable to bind oligonucleotides. That information is extremely important to design optimal sequences for antisense oligonucleotides and siRNA because both bind to single-stranded regions of target RNAs.

Aptamer microarray as a novel bioassay for protein-protein interaction discovery and analysis

Aptamer microarray is investigated as a novel bioassay for protein-protein interaction (PPI) discovery and analysis. Assaying a mixture of fluorescence-labeled thrombin and Escherichia coli proteins with an aptamer microarray, we found that thrombin and an unknown protein of E. coli (protein X) formed a complex of PPI, which was captured by an anti-thrombin aptamer probe. The PPI observed on the microarray was double-checked by protein microarrays and confirmed by aptamer-baited co-immunoprecipitation (Co-IP) assays. Characterizing the Co-IP products, we identified protein X as an E. coli Dps protein (DNA-binding protein from starved cells). A SDS-PAGE analysis suggested that Dps should be a substrate for thrombin, a trypsin-like serine protease. A dose-response microarray experiment predicted an apparent dissociation constant of 1.33 μM for the PPI. Moreover, an on-microarray competition assay revealed that the capture of the PPI by the anti-thrombin aptamer probe would be blocked by an E. coli aptamer via complementary base pairing. Thus, a network of protein-protein, protein-DNA, and DNA-DNA interactions and their interaction orders could be addressed in addition to simple PPI discovery.

Preparation and characterization of DNA aptamer based spin column for enrichment and separation of histones

Core histones play a critical role in the regulation of chromatin-templated biological process. Here we have developed a novel approach to enrich histone proteins based on the recognition of a specific aptamer. The method shows good selectivity toward histone proteins and is able to extract histones directly from complex whole cell lysates.