Colorimetric aptasensor for progesterone detection based on surfactant-induced aggregation of gold nanoparticles

A nitrogen-doped hollow carbon nanospheres-based aptasensor for non-invasive salivary detection of progesterone

Developing an easy-to-use and non-invasive sensor for monitoring progesterone (P4) as a multi-functional hormone is highly demanded for point-of-care testing. In this study, an ultrasensitive electrochemical aptasensor is fabricated for monitoring P4 in human biofluids. The sensing interface was designed based on the porous nitrogen-doped hollow carbon spheres (N-HCSs). The N-HCSs covalently immobilized high-dense aptamer (Apt) sequences as the bioreceptor of P4. The electron transfer of the redox probe was hindered by incubating P4 on the aptasensor surface and forming the P4-Apt complexes. Meanwhile, the signaling was decreased under two wide linear dynamic ranges (LDRs) from 10 fM to 5.6 μM with a limit of detection (LOD) value of 3.33 fM. The aptasensor presented satisfactory selectivity in the presence of different off-target species with successful feasibility for P4 detection in some human urine and saliva samples. The aptasensor with high sensitivity, as an advantage for on-site and sensitive measurement of P4, can be considered a non-invasive tool for routine analysis of real-world clinical samples method.

Antibacterial and antifungal action of CTAB-containing silica nanoparticles against human pathogens

New antibiotic agents are urgently needed worldwide to combat the increasing tolerance and resistance of pathogenic fungi and bacteria to current antimicrobials. Here, we looked at the antibacterial and antifungal effects of minor quantities of cetyltrimethylammonium bromide (CTAB), ca. 93.8 mg g^-1, on silica nanoparticles (MPSi-CTAB). Our results show that MPSi-CTAB exhibits antimicrobial activity against Methicillin-resistant Staphylococcus aureus strain (S. aureus ATCC 700698) with MIC and MBC of 0.625 mg mL^-1 and 1.25 mg mL^-1, respectively. Additionally, for Staphylococcus epidermidis ATCC 35984, MPSi-CTAB reduces MIC and MBC by 99.99% of viable cells on the biofilm. Furthermore, when combined with ampicillin or tetracycline, MPSi-CTAB exhibits reduced MIC values by 32- and 16-folds, respectively. MPSi-CTAB also exhibited in vitro antifungal activity against reference strains of Candida, with MIC values ranging from 0.0625 to 0.5 mg mL^-1. This nanomaterial has low cytotoxicity in human fibroblasts, where over 80% of cells remained viable at 0.31 mg mL^-1 of MPSi-CTAB. Finally, we developed a gel formulation of MPSi-CTAB, which inhibited in vitro the growth of Staphylococcus and Candida strains. Overall, these results support the efficacy of MPSi-CTAB with potential application in the treatment and/or prevention of infections caused by methicillin-resistant Staphylococcus and/or Candida species.

Colorimetric detection of Salmonella typhimurium based on hexadecyl trimethyl ammonium bromide-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles

We developed an effective and specific colorimetric strategy to detect Salmonella typhimurium (S. typhimurium) based on hexadecyl trimethyl ammonium bromide (CTAB)-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles (β-CD-AuNPs). In this study, ssDNA aptamer of S. typhimurium could combine with CTAB to form the supramolecular ssDNA-CTAB composite, so the ssDNA aptamer was applied to control the concentration of CTAB. In the presence of S. typhimurium, ssDNA aptamers selectively bound to S. typhimurium but not to CTAB, leading to the host-guest chemistry reaction of CTAB and β-CD resulting in β-CD-AuNP supramolecular assembly aggregation with an obvious color change. The ratio of absorption at 650 and 520 nm (A_650nm/A_520nm) has a linear correlation to the log scale of the concentration of the bacteria (1 × 10²-1 × 10⁷ CFU/mL) with a low limit of detection (LOD) of 13 CFU/mL. In addition, this optical sensor has good selectivity and practicability. In milk samples, the recovery was 93.55-111.32%, which suggested its potential application in real samples.

Monolayer-Protected Gold Nanoparticles Functionalized with Halogen Bonding Capability─An Avenue for Molecular Detection Schemes

The use of functionalized nanoparticles (NPs) and their aggregation in the presence of a targeted analyte is a well-established molecular detection strategy predicated on harnessing specific molecular interactions to the NP periphery. Molecules able to specifically interact with the functionalized NPs alter the unique optical and electrochemical properties of the NPs as a function of interparticle spacing. While many intermolecular interactions have been successfully exploited in this manner in conjunction with aqueous NP systems, the use of non-aqueous NPs in the same capacity is significantly less explored. A fundamental interaction that has not been previously investigated in NP schemes is halogen bonding (XB). XB is an orthogonal, electrostatic interaction between a region of positive electrostatic potential (δ+) on a halogen atom (i.e., XB donor) and a negative (δ-) Lewis base (XB acceptor) molecule. To couple XB with NP systems, ligands featuring a molecular structure that promotes XB interactions need to be identified, optimized, and synthesized for subsequent attachment to NPs. Herein, density functional theory (DFT) and NMR techniques are used to identify a strong XB-donor moiety (-C₆F₄I) and a synthetic scheme for a thiolate ligand featuring that functionality is devised and executed with high purity/yield (78%). Ligand-exchange reactions allow functionalization of non-aqueous alkanethiolate-protected gold NPs or monolayer-protected clusters (MPCs) with the XB-donor ligands. Functionalized MPCs (f-MPCs), within both assembled films and in solution, are shown to engage in XB interactions with target XB-acceptor molecules. Molecular recognition events, including induced aggregation of the f-MPCs, are characterized with UV-vis spectroscopy, cyclic voltammetry, TEM imaging, and diffusion-ordered spectroscopy NMR with limits of detection of 50-100 nM for strong XB acceptors. While fundamental exploration of XB interactions is ongoing, this study represents a step toward utilizing XB within molecular detection schemes, an application with implications for supramolecular chemistry, forensic, and environmental chemical sensing.

Colorimetric detection of ciprofloxacin in aqueous solution based on an unmodified aptamer and the aggregation of gold nanoparticles

A colorimetric method is described for detection of the antibiotic ciprofloxacin (CIP) in aqueous solutions based on an unmodified CIP-aptamer and gold nanoparticles (AuNPs), which are regarded as a probe and an indicator, respectively. In the absence of CIP, aptamers can hybridize with poly dimethyl diallyl ammonium chloride (PDDA) to form a 'duplex' structure through electrostatic interactions, and AuNPs remain dispersed in solution, leading to a red solution and an obvious absorption peak at 520 nm. Contrarily, the CIP-aptamer can specifically bind to CIP after the introduction of CIP into the solution. Then PDDA is thus free to aggregate AuNPs. The solution turns blue from red accordingly, presenting a strong absorption at 650 nm. Hence, the concentration of CIP can be quantified through the changes of the absorption. This sensitive and selective colorimetric method for CIP detection has a good linear response (R = 0.9935) in the 20 to 300 nM CIP concentration range, with a limit of detection (LOD) of 0.215 nM. Importantly, the proposed aptasensor demonstrates great application potential in CIP determination in aqueous samples.

An electrochemical biosensing platform for progesterone hormone detection using magnetic graphene oxide

In recent times, graphene and its derivatives have turned out to be emerging nanomaterials as transducers to promote electron transport in the field of biosensing using electrochemical techniques. In electrochemical biosensing strategies, key factors such as signal amplification, stability, and sensitivity are necessary for attaining improved sensor performance. In the present work, we synthesized magnetic nanocomposites of graphene oxide and employed them as an electrode material for the loading of bio receptors. The increased surface area with high electric conductance enhanced the sensor's response. The immobilization of progesterone (PGN) antibodies on the modified electrode-sensing surface led to a hindered electron transport that decreased the current response. The developed electrochemical immunosensor assembled successfully in a stepwise process using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies along with the electrochemical impedance spectroscopy (EIS) analysis. The current response decreased linearly with the increased progesterone (PGN) concentration range of 0.01 pM-1000 nM with excellent detection limits of 0.15 pM (DPV) and 0.17 pM (CV) under optimal experimental conditions. The label-free electrochemical immunosensor has shown a promising platform for rapid and direct analysis of PGN due to its high sensitivity, selectivity, stability, and repeatability in water samples.

Colorimetric Aptasensor for Testosterone Detection Based on Aggregation of Gold Nanoparticles Induced by Cationic Surfactant

This paper proposes a colorimetric aptasensor for the detection of testosterone (TES) in environmental water, using TES-specific aptamer (apT5) as a sensing probe, gold nanoparticles (AuNPs) as indicator, and hexadecyltrimethylammonium bromide (CTAB) as inducer, respectively. Based on competition between TES and CTAB for apT5, the aptamer can form an aptamer–TES complex, leaving CTAB free to aggregate AuNPs in the presence of TES. Dispersed and aggregated AuNPs have different absorption wavelengths and the signal of absorption intensity is associated with the concentration of TES, so TES can be detected quantitatively based on the signal absorption intensity. This sensitive aptasensor for TES detection has a wide linear range (R=0.998) from 1.91–800nM and a limit of detection (LOD) of 1.91nM. In addition, this aptasensor has high selectivity over some interferents. The method detects TES in tap water samples with recoveries in the range of 98.9–102.6% (RSD ≤ 7.35%). This biosensor presents a good and potential application to rapidly detect TES in actual environmental water samples.

Aptamer-Based Colorimetric Probe for trans-Zeatin Detection Using Unmodified Gold Nanoparticle

Trans-Zeatin is the major active phytohormone in immature corn kernels. Herein, a highly sensitive, good selective and simple aptamer-based colorimetric method for the detection of trans-zeatin was constructed. The selected aptamer sequence binds with trans-zeatin and induces a duplex-to-aptamer structure switching. The gold nanoparticles (AuNPs) solution is stable with high-concentration salt, which is protected by red complementary DNA. In the absence of trans-zeatin, the color of AuNPs changed from red to blue because aptamer DNA and complementary DNA form double-stranded DNA. Thus, the ratio of absorbance intensities (A522/A650) of AuNPs is changed with the concentration of trans-zeatin. The color change could be observed by the naked eye. The linear range of this method covers a large variation of trans-zeatin concentration from 0.05 to 0.75 μM. The detection limit is 0.037 μM. Moreover, this method was applied successfully to detect trans-zeatin in real plant samples.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2): a global pandemic and treatment strategies

The emergence and rapid spread of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), a potentially fatal disease, is swiftly leading to public health crises worldwide. The origin of SARS-CoV-2 infection was first reported in people exposed to a seafood market in Wuhan City, China in December 2019. It has been suggested that the infection is likely to be of zoonotic origin and transmitted to humans through a not-yet-known intermediary. As of 22 May 2020, the World Health Organization reported that there were approximately 4,995,996 confirmed cases and 327,821 deaths. SARS-CoV-2 is transmitted via inhalation or direct contact with droplets from infected people. It has an incubation period ranging from 2 to ≥14 days. The rate of spread of SARS-CoV-2 is greater than that for severe acute respiratory syndrome coronavirus and Middle East respiratory coronavirus. The symptoms are similar to influenza (i.e. breathlessness, sore throat and fatigue) and infected cases are isolated and treated. Infection is mild in most cases, but in elderly (>50 years) patients and those with cardiac and respiratory disorders, it may progress to pneumonia, acute respiratory distress syndrome and multi-organ failure. People with strong immunity or those who have developed herd immunity are asymptomatic. The fatality rate ranges from 3% to 4%. Recommended methods for diagnosis of COVID-19 are molecular tests (e.g. polymerase chain reaction) on respiratory secretions, chest scan and common laboratory diagnosis. Currently, treatment is essentially supportive, and the role of antiviral agents is yet to be established as a vaccine is not yet available. This review will focus on epidemiology, symptoms, transmission, pathogenesis, ongoing available treatments and future perspectives of SARS-CoV-2.

Discrimination of radiosensitive and radioresistant murine lymphoma cells by Raman spectroscopy and SERS

Intrinsic radiosensitivity is a biological parameter known to influence the response to radiation therapy in cancer treatment. In this study, Raman spectroscopy and surface enhanced Raman spectroscopy (SERS) were successfully used in conjunction with principal component analysis (PCA) to discriminate between radioresistant (LY-R) and radiosensitive (LY-S) murine lymphoma sublines (L5178Y). PCA results for normal Raman analysis showed a differentiation between the radioresistant and radiosensitive cell lines based on their specific spectral fingerprint. In the case of SERS with gold nanoparticles (AuNPs), greater spectral enhancements were observed in the radioresistant subline in comparison to its radiosensitive counterpart, suggesting that each subline displays different interaction with AuNPs. Our results indicate that spectroscopic and chemometric techniques could be used as complementary tools for the prediction of intrinsic radiosensitivity of lymphoma samples.

Preparation of DNA-functionalized surfaces for simultaneous homeotropic orientation of liquid crystals and optical recognition of analytes: application to the determination of progesterone

The work describes a simplified method for the preparation of liquid crystal (LC) bioassay using DNA-based capture molecules and having lower detection limits. The capture DNA probes of the stem-loop structure were immobilized on the surface of a glass slide. A homeotropic orientation of LC molecules can be obtained with the proper surface coverage of capture DNA probes. In the presence of analytes (specifically shown here for the progesterone as a model analyte), the molecular binding between capture DNA probes and progesterone opens the loop of the capture DNA probes. The opened sequence is then amenable to hybridization with a reporter DNA probe that is immobilized on gold nanoparticles. This changes the surface microstructure, disrupts the orientation of LC molecules, and results in an enhanced optical response, expressed as the average grey value of the images. This new kind of surface treatment for simultaneous recognition of target molecules and homeotropic anchoring of LCs reduces the number of preparation steps and makes the process of LC bioassay easier. This method has a detection limit as low as 0.1 pmol·L^-1 of progesterone. Graphical abstract Schematic presentation of the liquid crystal-based DNA assay. DMOAP: Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride; TEA: Triethoxsilylbutyraldehyde; 5CB: 4-cyano-4'-pentylbiphenyl; P4: progesterone.

Critical Review: DNA Aptasensors, Are They Ready for Monitoring Organic Pollutants in Natural and Treated Water Sources?

There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

Localized surface plasmon resonance-based abscisic acid biosensor using aptamer-functionalized gold nanoparticles

Abscisic acid (ABA) plays an important role in abiotic stress response and physiological signal transduction resisting to the adverse environment. Therefore, it is very essential for the quantitative detection of abscisic acid (ABA) due to its indispensable role in plant physiological activities. Herein, a new detection method based on localized surface plasmon resonance (LSPR) using aptamer-functionalized gold nanoparticles (AuNPs) is developed without using expensive instrument and antibody. In the presence of ABA, ABA specifically bind with their aptamers to form the ABA-aptamer complexes with G-quadruplex-like structure and lose the ability to stabilize AuNPs against NaCl-induced aggregation. Meanwhile, the changes of the LSPR spectra of AuNP solution occur and therefore the detection of ABA achieved. Under optimized conditions, this method showed a good linear range covering from 5×10-7 M to 5×10-5 M with a detection limit of 0.33 μM. In practice, the usage of this novel method has been demonstrated by its application to detect ABA from fresh leaves of rice with the relative error of 6.59%-7.93% compared with ELISA bioassay. The experimental results confirmed that this LSPR-based biosensor is simple, selective and sensitive for the detection of ABA. The proposed LSPR method could offer a new analytical platform for the detection of other plant hormones by changing the corresponding aptamer.

High affinity truncated DNA aptamers for the development of fluorescence based progesterone biosensors

Aptamers have shown a number of potential applications in sensing and therapeutic due to the high affinity and specificity towards their target molecules. Not all the nucleotides in the full length aptamers are involved in the binding with their targets. The non-binding domain of the aptamer may affect the binding affinity of the aptamer-target complex. Mapping the aptamer binding region could increase the affinity and the specificity. In this paper, we designed aptamer-based fluorescence sensors from a truncated progesterone (P4) aptamer. Then, fluorescein and quencher labelled aptamer complementary oligonucleotide sequences were hybridized to the truncated aptamer at different sites to form duplex structures. We used fluorescence-quencher pair displacement assay upon progesterone binding for the determination of P4. One of the truncated sequences has shown high binding affinity with 16 fold increase in the dissociation constant, K_d (2.1 nM) compared to the original aptamer. The aptasensor was highly selective for P4 against similar compounds such as 17-β estradiol, bisphenol-A and vitamin D. The sensor has been applied for the detection of P4 in spiked tap water and in urine samples showing good recovery. This new developed aptamer-based fluorescence biosensor can be applied in food, pharmaceutical, and clinical industries.

Fluorescent aptasensor for 17β-estradiol determination based on gold nanoparticles quenching the fluorescence of Rhodamine B

In this paper, we developed a fluorescent aptasensor for 17β-estradiol (E2) determination in aqueous solution using label-free E2-specific aptamer, gold nanoparticles (AuNPs) and Rhodamine B (RhoB) as sensing probe, fluorescent quencher and fluorescent indicator respectively. In the absence of E2, AuNPs were wrapped by E2 aptamer and maintained dispersed in NaCl solution basically. These dispersed AuNPs could effectively impair the originally high fluorescence of RhoB. Contrarily, in the presence of E2, E2 aptamer could specifically combine with E2 to form E2-aptamer complex, so the AuNPs were released by E2 aptamer and aggregated under the influence of NaCl. The aggregated AuNPs have a weak influence on RhoB fluorescence. Therefore, the E2 concentration can be determined by the change of fluorescence intensity of RhoB. This fluorescent assay has a detection limit as low as 0.48 nM, a linear range from 0.48 to 200 nM, and high selectivity over other disrupting chemicals. It was applied to determine E2 in water samples with recoveries in the range of 94.3-111.7%. The fluorescent aptasensor holds great potential for E2 detection in environmental water samples.