Optical Sensors Based on Hybrid Aptamer/Conjugated Polymer Complexes

Reduced graphene oxide as a resonance light-scattering probe for thrombin detection using dual-aptamer-based dsDNA

This paper describes a reduced graphene oxide (RGO)-based resonance light-scattering (RLS) method for thrombin detection by using double strand DNA (dsDNA) as a binding element. dsDNA is obtained by hybridizing DNA1 and DNA2, which respectively consist of one aptamer of thrombin and the complementary strand of the other aptamer of thrombin. When thrombin is added, the specific binding of two aptamers to thrombin results in a complex (DNA1-thrombin-DNA2) and triggers the release of the complementary strand of two aptamers from dsDNA. The released ssDNA can be self-assembled on the surface of RGO to form a stable DNA1-thrombin-DNA2-RGO complex, which increases RLS signals. This simple and rapid method has enabled the detection of thrombin in the picomolar level in buffer and human serum samples. This study is the first to use RGO as a platform in RLS sensor, which can extend the application of RGO.

A universal aptameric biosensor: Multiplexed detection of small analytes via aggregated perylene-based broad-spectrum quencher

A universal aptameric system based on the taking advantage of double-stranded DNA/perylene diimide (dsDNA/PDI) as the signal probe was developed for multiplexed detection of small molecules. Aptamers are single-stranded DNA or RNA oligonucleotides which are selected in vitro by a process known as systematic evolution of ligands by exponential enrichment. In this work, we synthesized a new kind of PDI and reported this aggregated PDI could quench the double-stranded DNA (dsDNA)-labeled fluorophores with a high quenching efficiency. The quenching efficiencies on the fluorescence of FAM, TAMRA and Cy5 could reach to 98.3%±0.9%, 97.2%±0.6% and 98.1%±1.1%, respectively. This broad-spectrum quencher was then adopted to construct a multicolor biosensor via a label-free approach. A structure-switching-triggered enzymatic recycling amplification was employed for signal amplification. High quenching efficiency combined with autocatalytic target recycling amplification afforded the biosensor with high sensitivity towards small analytes. For other targets, changing the corresponding aptamer can achieve the goal. The quencher did not interfere with the catalytic activity of nuclease. The biosensor could be manipulated with similar sensitivity no matter in pre-addition or post-addition manner. Moreover, simultaneous and multiplexed analysis of several small molecules in homogeneous solution was achieved, demonstrating its potential application in the rapid screening of multiple biotargets.

Gold nanrods plasmon-enhanced photoelectrochemical aptasensing based on hematite/N-doped graphene films for ultrasensitive analysis of 17β-estradiol

It remains a vital task to establish ultrasensitive sensing interfaces for detection of target analytes to meet the demands of modern analysis. Herein, a highly sensitive turn-on photoelectrochemical (PEC) platform for trace 17β-estradiol (E2) assay was developed based on Au nanrods (AuNRs) with surface plasmon resonance (SPR) properties induced signal amplification. Specifically, a ternary hybrid was prepared by integrating hematite (α-Fe₂O₃) nanocrystals and N-doped graphene (NG) with AuNRs, which further served as highly efficient photoactive species. Subsequently, a PEC sensing platform was fabricated based on the specific binding of E2 and its aptamer. On such a sensor, the capture of E2 molecules by aptamers led to increased photocurrent. This was attributed to that the specific recognition reaction between E2 and aptamer resulted in the conformational change of the aptamers and complete dissociation of some aptamers on the PEC sensing interface. It can be confirmed by the electrochemical impedance spectroscopy (EIS) results. This process decreased the steric hindrances between the electrode surface and solution and thus increased the photocurrent response. Under the optimal conditions, the as-prepared PEC aptasensor exhibited superb analytical performances for detection of E2 in the range from 1×10^-15M to 1×10^-9M with a detection limit of 3.3×10^-16M. The aptasensor manifested outstanding selectivity towards E2 when other endocrine disrupting compounds with similar structure coexisted. Furthermore, the aptasensor was successfully applied for the determination of E2 in milk powder. The present strategy provides a potential way to boost the activity of photoactive materials and improve the sensitivity of PEC biosensor.

Salt-induced thermochromism of a conjugated polyelectrolyte

We report here the photophysical properties of a water-soluble polythiophene with cationic side-chains in PBS buffer solution.

Label-Free Aptasensors for the Detection of Mycotoxins

Various methodologies have been reported in the literature for the qualitative and quantitative monitoring of mycotoxins in food and feed samples. Based on their enhanced specificity, selectivity and versatility, bio-affinity assays have inspired many researchers to develop sensors by exploring bio-recognition phenomena. However, a significant problem in the fabrication of these devices is that most of the biomolecules do not generate an easily measurable signal upon binding to the target analytes, and signal-generating labels are required to perform the measurements. In this context, aptamers have been emerged as a potential and attractive bio-recognition element to design label-free aptasensors for various target analytes. Contrary to other bioreceptor-based approaches, the aptamer-based assays rely on antigen binding-induced conformational changes or oligomerization states rather than binding-assisted changes in adsorbed mass or charge. This review will focus on current designs in label-free conformational switchable design strategies, with a particular focus on applications in the detection of mycotoxins.

Aptamer-Functionalized Fluorescent Silica Nanoparticles for Highly Sensitive Detection of Leukemia Cells

A simple, highly sensitive method to detect leukemia cells has been developed based on aptamer-modified fluorescent silica nanoparticles (FSNPs). In this strategy, the amine-labeled Sgc8 aptamer was conjugated to carboxyl-modified FSNPs via amide coupling between amino and carboxyl groups. Sensitivity and specificity of Sgc8-FSNPs were assessed using flow cytometry and fluorescence microscopy. These results showed that Sgc8-FSNPs detected leukemia cells with high sensitivity and specificity. Aptamer-modified FSNPs hold promise for sensitive and specific detection of leukemia cells. Changing the aptamer may allow the FSNPs to detect other types of cancer cells.

The aptamer DNA-templated fluorescence silver nanoclusters: ATP detection and preliminary mechanism investigation

Two general and reliable fluorescence sensors were proposed in this work utilizing aptamer DNA-templated silver nanoclusters (Ag NCs). Both DNA-AgNCs could be used for label-free detecting of ATP with the limits of detection of 0.44 and 0.65mM. One of them was further applied to monitor the activity of adenosine deaminase (ADA). In our effort to elucidate the light-up mechanism, we studied a total of six Ag NCs prepared by different DNA sequences, and found that they showed different sensitivity to ATP. Both BT3T3- and BT3T3(R)-templated Ag NCs were chose to make particular studies by UV-vis, TEM, fluorescence, and TCSPC methods. The results showed that when DNA-Ag NCs was kept for 1.5h and presented a strong fluorescence, the addition of ATP failed to cause a large change of fluorescence intensity; on the contrary, after Ag NCs was kept for 24h and emitted a weak fluorescence, adding ATP was able to result in the large fluorescence enhanced of 43 and 33 times for BT3T3- and BT3T3(R)-templated Ag NCs, respectively. The possible mechanism was also suggested that ATP binding to aptamer segment of template induced the change of the DNA secondary structure, which made the aggregated Ag nanoparticles disperse into Ag NCs with an average diameter of about 2nm that were responsible for the large fluorescence increase. Moreover, ATP could protect the fluorescence intensity of BT3T3(R)-templated Ag NCs from quenching for at least 9h.

Surfactant-Triggered Fluorescence Turn "on/off" Behavior of a Polythiophene-graft-Polyampholyte

Polythiophene-graft-polyampholyte (PTP) is synthesized using N,N-dimethylaminoethyl methacrylate and tert-butyl methacrylate monomers by grafting from polythiophene backbone, followed by hydrolysis. The resulting polymer exhibits aqueous solubility via formation of small-sized miceller aggregates with hydrophobic polythiophene at the center and radiating polyionic side chains (cationic or anionic depending on the pH of the medium) at the outer periphery. The critical micelle concentration of PTP in acidic solution (0.025 mg/mL, pH = 2.7) is determined from fluorescence spectroscopy. PTP exhibits reversible fluorescence on and off response in both acidic and basic medium with the sequential addition of differently charged ionic surfactants, repeatedly. The fluorescence intensity of PTP at pH 2.7 increases with the addition of an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS), due to the self-aggregation forming compound micelles. The fluorescence intensity of these solutions again decreases on addition of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), because of assembling of SDBS with CTAB, thus deassembling the PTP-SDBS aggregates. At pH 9.2, these turn on and turn off responses are also shown by PTP with the sequential addition of cationic surfactant (CTAB) and anionic surfactant (SDBS), respectively. This result shows that PTP has potential for surfactant-induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through self-assembling and deassembling of the ionic aggregates. The reversible aggregation and disaggregation process of PTP with the surfactants at both acidic and basic pH is supported from dynamic light scattering and Fourier transform infrared spectroscopy. The morphology of the above systems studied by transmission and scanning electron microscopy also supports the above aggregation and disaggregation process.

Simple and rapid chemiluminescence aptasensor for Hg2+ in contaminated samples: A new signal amplification mechanism

Detection of ultralow concentration of heavy metal ion Hg²⁺ is important for human health protection and environment monitoring because of the gradual accumulation in environmental and biological fields. Herein, we report a convenient chemiluminescence (CL) biosensing platform for ultrasensitive Hg²⁺ detection by signal amplification mechanism from positively charged gold nanoparticles ((+)AuNPs). It is based on (+)AuNPs charge effect and aptamer conformation change induced by target to stimulate the generation of CL in the presence of H₂O₂ and luminol without high salt medium. Notably particularly, the typical problem of the high salt medium from (-) AuNPs system, like influencing aptamers' bind with target and hindering CL reaction can be effectively addressed through the direct introduction of (+)AuNPs. Therefore, the proposed biosensing exhibits a high sensitivity toward target Hg²⁺ with a detection limit of 16 pM, which is far below the limit (10nM) defined by the U.S. Environmental Protection Agency in drinkable water, and is about 10-fold lower than the previously reported aptamer-based assays for Hg²⁺. This sensing platform provides a simple, rapid, and cost-effective approach for label-free sensitive detection of Hg²⁺. Moreover, it is universal for the detection of other targets. Undoubtedly, such a direct utilizing of (+)AuNPs' charge effect will provide a new signal amplification way for label-free aptamer-based CL analysis.

Visual detection of thrombin using a strip biosensor through aptamer-cleavage reaction with enzyme catalytic amplification

A new class of strip biosensors has been established based on well-distributed thrombin aptamer-linked gold nanoparticle aggregates, which will undergo a cracking reaction when the target recognizes its homologous aptamer. Combining the aptamer-cleavage reaction with the enzyme catalytic amplification system, our proposed lateral flow strip biosensor (LFB) is capable of visually detecting 6.4 pM of thrombin without instrumentation within 12 minutes. Under the optimal conditions, the quantitative detection of thrombin by a portable strip reader exhibited a linear relationship between the peak area and the concentration of thrombin in the range of 6.4 pM-500 nM with a detection limit of 4.9 pM, which is three orders of magnitude lower than that of the aptamer-functionalized gold nanoparticle-based LFB (2.5 nM, Xu et al., Anal. Chem., 2009, 81, 669-675). As the aptamers have no special requirements and the gold nanoparticles can also be replaced by other metallic nanoparticles, this method for strip sensing is expected to be generally applicable in point of care testing, home testing, medical diagnostics, clinical diagnosis, and environmental monitoring.

Oligonucleotide-based label-free detection with optical microresonators: strategies and challenges

This review targets diversified oligonucleotide-based biodetection techniques, focusing on the use of microresonators of whispering gallery mode (WGM) type as optical biosensors mostly integrated with lab-on-a-chip systems. On-chip and microfluidics combined devices along with optical microresonators provide rapid, robust, reproducible and multiplexed biodetection abilities in considerably small volumes. We present a detailed overview of the studies conducted so far, including biodetection of various oligonucleotide biomarkers as well as deoxyribonucleic acids (DNAs), ribonucleic acids (RNAs) and proteins. We particularly advert to chemical surface modifications for specific and selective biosensing.

Robust aptamer-polydopamine-functionalized M-PLGA-TPGS nanoparticles for targeted delivery of docetaxel and enhanced cervical cancer therapy

One limitation of current biodegradable polymeric nanoparticles (NPs) is the contradiction between functional modification and maintaining formerly excellent bioproperties with simple procedures. Here, we reported a robust aptamer-polydopamine-functionalized mannitol-functionalized poly(lactide-co-glycolide) (M-PLGA)-D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) nanoformulation (Apt-pD-NPs) for the delivery of docetaxel (DTX) with enhanced cervical cancer therapy effects. The novel DTX-loaded Apt-pD-NPs possess satisfactory advantages: 1) increased drug loading content and encapsulation efficiency induced by star-shaped copolymer M-PLGA-TPGS; 2) significant active targeting effect caused by conjugated AS1411 aptamers; and 3) excellent long-term compatibility by incorporation of TPGS. Therefore, with simple preparation procedures and excellent bioproperties, the new functionalized Apt-pD-NPs could maximally increase the local effective drug concentration on tumor sites, achieving enhanced treatment effectiveness and minimizing side effects. In a word, the robust DTX-loaded Apt-pD-NPs could be used as potential nanotherapeutics for cervical cancer treatment, and the aptamer-polydopamine modification strategy could be a promising method for active targeting of cancer therapy with simple procedures.

A simple and rapid chemiluminescence aptasensor for acetamiprid in contaminated samples: Sensitivity, selectivity and mechanism

Ultralow concentration and selective detection of pesticide residue is important to evaluate the environmental and biological pollution and the threat to human health which single component pesticide can bring. Herein, we report an amplified chemiluminescence (CL) sensing platform for ultrasensitive and selective acetamiprid (widely used pesticide) detection. It is based on aptamer's high binding affinity to target and the relevance between AuNPs' morphology and its catalytic effect to stimulate the generation of CL in the presence of H2O2 and luminol. Moreover, AuNPs morphological slight change induced by aptamers' conformation during targets binding could lead to the significant change of catalytic properties. Therefore, the proposed sensing platform for pesticide residue exhibited a high sensitivity toward acetamiprid with a detection limit of 62pM, which was about 100-fold lower than that of other aptamer-based sensor for acetamiprid detection. Because of the intrinsic specificity of aptamer's recognization, this sensing platform has high selectivity. So, this sensing platform provides a label-free and cost-effective approach for sensitive and selective detection of single component pesticide residue. More importantly, this CL method was successfully used to determine acetamiprid in real contaminated samples.

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.

A G-quadruplex-based Label-free Fluorometric Aptasensor for Adenosine Triphosphate Detection

A G-quadruplex-based, label-free fluorescence assay was demonstrated for the detection of adenosine triphosphate (ATP). A double-stranded DNA (dsDNA), hybridized by ATP-aptamer and its complementary sequence, was employed as a substrate for ATP binding. SYBR Green I (SG I) was a fluorescent probe and exonuclease III (Exo III) was a nuclease to digest the dsDNA. Consequently, in the absence of ATP, the dsDNA was inset with SG I and was digested by Exo III, resulting in a low background signal. In the presence of ATP, the aptamer in dsDNA folded into a G-quadruplex structure that resisted the digestion of Exo III. SG I was inserted into the structure, showing high fluorescence. Owing to a decrease of the background noise, a high signal-to-noise ratio could be obtained. This sensor can detect ATP with a concentration ranging from 50 μM to 5 mM, and possesses a capacity for the sensitive determination of other targets.

Aptamers in analytics

Nucleic acid aptamers are promising alternatives to antibodies in analytics. They are generally obtained through an iterative SELEX protocol that enriches a population of synthetic oligonucleotides to a subset that can recognize the chosen target molecule specifically and avidly. A wide range of targets is recognized by aptamers. Once identified and optimized for performance, aptamers can be reproducibly synthesized and offer other key features, like small size, low cost, sensitivity, specificity, rapid response, stability, and reusability. This makes them excellent options for sensory units in a variety of analytical platforms including those with electrochemical, optical, and mass sensitive transduction detection. Many novel sensing strategies have been developed by rational design to take advantage of the tendency of aptamers to undergo conformational changes upon target/analyte binding and employing the principles of base complementarity that can drive the nucleic acid structure. Despite their many advantages over antibodies, surprisingly few aptamers have yet been integrated into commercially available analytical devices. In this review, we discuss how to select and engineer aptamers for their identified application(s), some of the challenges faced in developing aptamers for analytics and many examples of their reported successful performance as sensors in a variety of analytical platforms.

Polydopamine-Based Surface Modification of Novel Nanoparticle-Aptamer Bioconjugates for In Vivo Breast Cancer Targeting and Enhanced Therapeutic Effects

In this study, we reported a simple polydopamine (pD)-based surface modification method to prepare novel nanoparticle-aptamer bioconjugates (Apt-pD-DTX/NPs) for in vivo tumor targeting and enhanced therapeutic effects of breast cancer. With simple preparation procedures, the new functionalized Apt-pD-DTX/NPs could maximumly increase the local effective drug concentration on tumor sites, achieving enhanced treatment effectiveness and minimizing side effects. The dopamine polymerization and aptamer conjugation barely changed the characters of NPs. Both in vitro cell experiments (i.e. endocytosis of fluorescent NPs, in vitro cellular targeting and cytotoxicity assays) and in vivo animal studies (i.e. in vivo imaging, biodistribution and antitumor effects of NPs) demonstrated that the Apt-pD-DTX/NPs could achieve significantly high targeting efficiency and enhanced therapeutic effects compared with clinical Taxotere(®) and NPs without functional modification. Above all, the Apt-pD-DTX/NPs showed great potential as a promising nanoformulation for in vivo breast cancer therapy and the construction of pD-modified NP-aptamer bioconjugates could be of great value in medical use.

Fluorescence and colorimetric detection of ATP based on a strategy of self-promoting aggregation of a water-soluble polythiophene derivative

A sensitive fluorescent and colorimetric dual-modal probe for the detection of ATP has been developed based on a strategy of self-promoting aggregation of a cationic polythiophene derivative bearing anthracene groups in the side chain with a detection limit as low as 10(-9) M.

High-throughout identification of telomere-binding ligands based on photo-induced electron transfer

A fast and cost-effective method is developed for high-throughout screening G-quadruplex-binding ligands based on the photo-induced electron transfer.

Label-Free and Sensitive Detection of BRCA1 and TB4 DNA Sequences with Water-Soluble Cationic Polythiophenes

A sensitive method for BRCA1 and TB4 DNA sequences detection using water-soluble cationic polythiophenes, poly(3-(1′-ethoxy-2′-N-methyl imidazole)thiophene) (PT) is presented. The fluorescence of PT could be dramatically quenched by the addition of single-stranded DNA (ssDNA; TB4 and BRCA1 sequences). The KSV (Stern–Volmer constant) for TB4 and BRCA1 DNA sequences are 1.46 × 108 and 3.28 × 108 M–1 respectively, and the limits of detection of these two sequences are 1.26 × 10–10 and 0.483 × 10–10 M respectively. The fluorescence of PT would recover to its initial intensity after the addition of complementary ssDNA, whereas sequences with one to three mismatched bases would not. The influences of buffer pH and concentration of NaCl were also investigated systemically in order to further improve the sensitivity. This assay can be completed in a short period of time without any further procedure. Hence, this sensitive, cost-effective, and rapid detection method for BRCA1 and TB4 DNA sequences may contribute to the clinical diagnosis of breast cancer gene mutations in the future.

Label-free DNA-based biosensors using structure-selective light-up dyes

Label-free biosensors (LFBs) have demonstrated great potential in cost-effective applications. This review collected the latest reported works which employed structure-selective nucleic acid dyes for the development of DNA-based LFBs.

Thin Films Formed from Conjugated Polymers with Ionic, Water-Soluble Backbones

This paper compares the morphologies of films of conjugated polymers in which the backbone (main chain) and pendant groups are varied between ionic/hydrophilic and aliphatic/hydrophobic. We observe that conjugated polymers in which the pendant groups and backbone are matched, either ionic-ionic or hydrophobic-hydrophobic, form smooth, structured, homogeneous films from water (ionic) or tetrahydrofuran (hydrophobic). Mismatched conjugated polymers, by contrast, form inhomogeneous films with rough topologies. The polymers with ionic backbone chains are conjugated polyions (conjugated polymers with closed-shell charges in the backbone), which are semiconducting materials with tunable bad-gaps, not unlike uncharged conjugated polymers.

A fluorescent aptasensor using double-stranded DNA/graphene oxide as the indicator probe

We developed a fluorescent aptasensor based on the making use of double-stranded DNA (dsDNA)/graphene oxide (GO) as the signal probe and the activities of exonuclease I (Exo I). This method takes advantage of the stronger affinity of the aptamer to its target rather than to its complementary sequence (competitor), and the different interaction intensity of dsDNA, mononucleotides with GO. Specifically, in the absence of target, the competitor hybridizes with the aptamer, preventing the digestion of the competitor by Exo I, and thus the formed dsDNA is adsorbed on GO surface, allowing fluorescence quenching. When the target is introduced, the aptamer preferentially binds with its target. Thereby, the corresponding nuclease reaction takes place, and slight fluorescence change is obtained after the introduction of GO due to the weak affinity of the generated mononucleotides to GO. Adenosine (AD) was chosen as a model system and tested in detail. Under the optimized conditions, smaller dissociation constant (Kd, 311.0 µM) and lower detection limit (LOD, 3.1 µM) were obtained in contrast with traditional dye-labeled aptamer/GO based platform (Kd=688.8 µM, LOD=21.2 µM). Satisfying results were still obtained in the evaluation of the specificity and the detection of AD in human serum, making it a promising tool for the diagnosis of AD-relevant diseases. Moreover, we demonstrated the effect of the competitor on the LOD, and the results reveal that the sensitivity could be enhanced by using the rational competitor. The present design not only constructs a label-free aptamer based platform but also extends the application of dsDNA/GO complex in biochemical and biomedical studies.

Nucleic acid aptamers in cancer research, diagnosis and therapy

Aptamers are single-stranded DNA or RNA oligomers, identified from a random sequence pool, with the ability to form unique and versatile tertiary structures that bind to cognate molecules with superior specificity. Their small size, excellent chemical stability and low immunogenicity enable them to rival antibodies in cancer imaging and therapy applications. Their facile chemical synthesis, versatility in structural design and engineering, and the ability for site-specific modifications with functional moieties make aptamers excellent recognition motifs for cancer biomarker discovery and detection. Moreover, aptamers can be selected or engineered to regulate cancer protein functions, as well as to guide anti-cancer drug design or screening. This review summarizes their applications in cancer, including cancer biomarker discovery and detection, cancer imaging, cancer therapy, and anti-cancer drug discovery. Although relevant applications are relatively new, the significant progress achieved has demonstrated that aptamers can be promising players in cancer research.

Superior fluorescent probe for detection of potassium ion

Here, a simple, and highly sensitive fluorescent assay is designed to monitor K(+). The versatile, robust biosensing strategy is based on the specific recognition utility of label-free aptamers with their targets and PicoGreen dye as the signal probe. The aptamers undergo a conformational change to a secondary structure such as G-quadruplex in the presence of targets. In addition to a conformational change with its targets, the remaining single-stranded DNA (ssDNA) aptamer form a duplex structure with its complete complementary sequence. Conformational changes of aptamers as well as fluorescence amplification produce clear signal-off in the presence of targets. Fluorescent assay employing this mechanism for the detection of K(+) is highly sensitive, and selective. The detection limit of the K(+) assay is determined to be 2.37 pM. The sensing strategy is low-cost and simple in its operation without requirement for complex labeling of probe DNA or sophisticated synthesis of the fluorescent compound. Also, the method has less structural requirement of complexes of aptamers with their targets, thus rending its wilder applications for various targets.

An aptamer-based single particle method for sensitive detection of thrombin using fluorescent quantum dots as labeling probes

In this study, an aptamer-based single particle method was developed for the thrombin detection in human serum samples using fluorescence correlation spectroscopy (FCS). In this method, quantum dots (QDs) were used as the fluorescent probes and thrombin-binding aptamer (TBA) was used as molecular recognition unit. When two QDs probes labeled with TBA (QD-TBA1 and QD-TBA2) are mixed in a sample containing thrombin targets, the binding of targets will cause QDs to form dimers (or oligomers) with bigger sizes, which leads to the nearly double increase in the characteristic diffusion time of QDs in the detection volume of FCS. FCS method can detect the change in the characteristic diffusion time of QDs. Firstly, the diffusion and blinking behaviors of QD-TBA probes in the presence of thrombin were investigated by FCS and total internal reflection fluorescence microscopy (TIRFM) imaging system, and the experimental results documented that QD-TBAs were bound together with "one-by-one" structure when thrombin were added into the solution. And then, the assay conditions were optimized in order to improve the sensitivity and specificity of this method. Under the optimized conditions, the linear range of the method is from 5.0 nM to 500 nM of thrombin, and the limit of detection is about 2.6 nM. Finally, this method was applied to homogeneous determination of thrombin in human serum samples.

Polycation-sodium lauryl ether sulfate-type surfactant complexes: influence of ethylene oxide length

Polyelectrolyte-surfactant complexes (PESC) are a class of materials which form spontaneously by self-assembly driven by electrostatic and hydrophobic interactions. PESC containing sodium lauryl ether sulfates (SLES) have found wide application in hair care products like shampoo. Typically, SLES with only one or two ethylene oxide (EO) groups are used for this application. We have studied the influence of the size of the EO block (ranging from 0 to 30 EO groups) on complexation with two model polycations: linear polyDADMAC and branched PEI. PESC size and electrostatic properties were determined during stepwise titration of buffered polycation solutions. The critical aggregation concentration (CAC) of PESC was determined by surface tension measurements and fluorescence spectroscopy. For polyDADMAC, there is no influence of the size of the EO block on the complexation behavior; the stiff polycation governs the structure formation. For PEI, it was seen that the EO block size does affect the structure of the complexes. The CAC value of the investigated complexes turns out to be rather independent of the EO block size; however, the CMC/CAC ratio decreases with increasing size of the EO block. This latter observation explains why the Lochhead-Goddard effect is most effective for small EO blocks.

Graphene oxide-assisted nucleic acids assays using conjugated polyelectrolytes-based fluorescent signal transduction

In this work, we investigated the interactions between graphene oxide (GO) and conjugated polyelectrolytes (CPEs) with different backbone and side chain structures. By studying the mechanism of fluorescence quenching of CPEs by GO, we find that the charge and the molecular structure of CPEs play important roles for GO-CPEs interactions. Among them, electrostatic interaction, π-π interaction, and cation-π bonding are dominant driving forces. By using a cationic P2, we have developed a sensitive homogeneous sensor for DNA and RNA detection with a detection limit of 50 pM DNA and RNA, which increased the sensitivity by 40-fold as compared to GO-free CPE-based sensors. This GO-assisted CPE sensing strategy is also generic and shows a high potential for biosensor designs based on aptamers, proteins, peptides, and other biological probes.

Differential vital staining of normal fibroblasts and melanoma cells by an anionic conjugated polyelectrolyte

Molecular probes for imaging of live cells are of great interest for studying biological and pathological processes. The anionic luminescent conjugated polythiophene (LCP) polythiophene acetic acid (PTAA), has previously been used for vital staining of cultured fibroblasts as well as transformed cells with results indicating differential staining due to cell phenotype. Herein, we investigated the behavior of PTAA in two normal and five transformed cells lines. PTAA fluorescence in normal cells appeared in a peripheral punctated pattern whereas the probe was more concentrated in a one-sided perinuclear localization in the five transformed cell lines. In fibroblasts, PTAA fluorescence was initially associated with fibronectin and after 24 h partially localized to lysosomes. The uptake and intracellular target in malignant melanoma cells was more ambiguous and the intracellular target of PTAA in melanoma cells is still elusive. PTAA was well tolerated by both fibroblasts and melanoma cells, and microscopic analysis as well as viability assays showed no signs of negative influence on growth. Stained cells maintained their proliferation rate for at least 12 generations. Although the probe itself was nontoxic, photoinduced cellular toxicity was observed in both cell lines upon irradiation directly after staining. However, no cytotoxicity was detected when the cells were irradiated 24 h after staining, indicating that the photoinduced toxicity is dependent on the cellular location of the probe. Overall, these studies certified PTAA as a useful agent for vital staining of cells, and that PTAA can potentially be used to study cancer-related biological and pathological processes.

An electrochemical DNA sensor based on polyaniline/graphene: high sensitivity to DNA sequences in a wide range

In the platform constructed by a polyaniline/graphene composite, double-stranded DNA, resulting from hybridization of a DNA probe, was escaping from or remaining on the sensor surface.

Cationic polythiophene for specific detection of cyanide ions in water using fluorometric technique

A new fluorometric cyanide sensor using cationic polythiophene exhibits high sensitivity, selectivity with a low detection limit (4.4 ppb) in water.

Multi-responsive behavior of highly water-soluble poly(3-hexylthiophene)-block-poly(phenyl isocyanide) block copolymers

Highly water-soluble block copolymers containing conjugated poly(3-hexylthiophene) and hydrophilic poly(phenyl isocyanide) segments were found to be responsive to multiple stimuli, such as temperature, pH and solvents.

Poly(aryleneethynylene)s (PAE) as paradigmatic sensor cores

What you need to know about poly(aryleneethynylene)s as sensory materials. A tutorial of fundamental properties and new developments since 2009.

Water soluble polythiophenes: preparation and applications

Different synthetic procedures for water soluble polythiophenes and their applications in sensing, detection of biomolecules and optoelectronic devices are discussed.