Semiconducting polymer dots as fluorescent probes for in vitro biosensing

Semiconducting polymer dots (Pdots) have emerged as novel fluorescent probes with excellent characteristics, such as ultrahigh molar extinction coefficient, easy tunable absorption and emission bands, high brightness, and excellent photostability. Combined with good biocompatibility properties, much effort has been devoted to Pdots for in vivo biological imaging and therapy applications, such as deep-tissue fluorescent imaging, photodynamic therapy, photothermal therapy, and nanocarriers of genes or chemical drugs. Many reviews have been presented in these fields. On the other hand, a large number of studies employing Pdots for in vitro biosensing applications have been reported during the past few years, and there are barely any relevant reports to summarize the progress in this area. Hence, it is necessary to review these studies to promote the comprehensive application of Pdots. Herein, we introduce the properties and functionalization of Pdots, and systematically summarize the progress in the in vitro applications of Pdots, including the detection of DNAs, microRNAs, proteins, enzymatic activity, and some biological small molecules and ions. Finally, we share our perspectives on the future direction of this field.

Selective mercury(ii) detection in aqueous solutions upon the absorption changes corresponding to the transition moments polarized along the short axis of an azobenzene chemosensor

A completely water-soluble azobenzene chemosensor shows selective Hg²⁺ detection properties in wide pH ranges and under different light conditions.

Thioflavin T as an Efficient G-Quadruplex Inducer for the Highly Sensitive Detection of Thrombin Using a New Föster Resonance Energy Transfer System

We report a new Föster resonance energy transfer (FRET) system that uses a special dye, thioflavin T (ThT), as an energy acceptor and a water-soluble conjugated polymer (CP) with high fluorescence as an energy donor. A simple, label-free, and sensitive strategy for the detection of thrombin in buffer and in diluted serum was designed based on this new system using ThT as an efficient inducer of the G-quadruplex. The difference between the blank and the positive samples was amplified due to distinctive FRET signals because thrombin has little effect on the intercalation of ThT into the G-quadruplex. In the absence of the target, ThT induces the aptamer to form a G-quadruplex and intercalates into it with strong fluorescence. The electrostatic attractions between the negatively charged G-quadruplex and positively charged CP allow a short donor-acceptor distance, resulting in a high FRET signal. However, in the presence of the target, the aptamer forms a G-quadruplex-thrombin complex first, followed by the intercalation of ThT into the G-quadruplex. A long distance exists between the donor and acceptor due to the strong steric hindrance from the large-sized thrombin, which leads to a low FRET signal. Compared with previously reported strategies based on the FRET between the CP and dye, our strategy is label-free, and the sensitivity was improved by an order of magnitude. Our strategy also shows the advantages of being simple, rapid (about 50 min), sensitive, label-free, and low-cost in comparison to strategies based on the FRET between quantum dots and dyes.

Nanomaterial-based approaches for the detection and speciation of mercury

Detection and speciation of Hg through the T–Hg–T coordination, Hg–S and/or Hg–Au/Ag interactions based colorimetric or fluorescent changes.

Carbamodithioate-based dual functional fluorescent probe for Hg(2+) and S(2-)

Carbamodithioate-based compound T1 was designed and synthesized as a dual-functional probe for Hg(2+) ions and S(2-) anions. The underlying signaling mechanism was intramolecular charge transfer (ICT). It could serve as a direct probe towards Hg(2+) ions through "on-off" fluorescence changes and an indirect probe towards S(2-) anions through "on-off-on" fluorescence changes.

Ratiometric fluorescent ion detection in water with high sensitivity via aggregation-mediated fluorescence resonance energy transfer using a conjugated polyelectrolyte as an optical platform

A cationic conjugated polyelectrolyte was designed and synthesized based on poly(fluorene-co-phenylene) containing 5 mol% benzothiadiazole (BT) as a low energy trap and 15-crown-5 as a recognizing group for potassium ions. A potassium ion can form a sandwich-type 2:1 Lewis acid-based complex with 15-crown-5, to cause the intermolecular aggregation of polymers. This facilitates inter-chain fluorescence resonance energy transfer (FRET) to a low-energy BT segment, resulting in fluorescent signal amplification, even at dilute analyte concentrations. Highly sensitive and selective detection of K(+) ions was demonstrated in water. The linear response of ratiometric fluorescent signal as a function of [K(+) ] allows K(+) quantification in a range of nanomolar concentrations with a detection limit of ≈0.7 × 10(-9) M.

Cationic conjugated polyelectrolytes-triggered conformational change of molecular beacon aptamer for highly sensitive and selective potassium ion detection

We demonstrate highly sensitive and selective potassium ion detection against excess sodium ions in water, by modulating the interaction between the G-quadruplex-forming molecular beacon aptamer (MBA) and cationic conjugated polyelectrolyte (CPE). The K(+)-specific aptamer sequence in MBA is used as the molecular recognition element, and the high binding specificity of MBA for potassium ions offers selectivity against a range of metal ions. The hairpin-type MBA labeled with a fluorophore and quencher at both termini undergoes a conformational change (by complexation with CPEs) to either an open-chain form or a G-quadruplex in the absence or presence of K(+) ions. Conformational changes of MBA as well as fluorescence (of the fluorophore in MBA) quenching or amplification via fluorescence resonance energy transfer from CPEs provide clear signal turn-off and -on in the presence or absence of K(+). The detection limit of the K(+) assays is determined to be ~1.5 nM in the presence of 100 mM Na(+) ions, which is ~3 orders of magnitude lower than those reported previously. The successful detection of 5'-adenosine triphosphate (ATP) with the MBA containing an ATP-specific aptamer sequence is also demonstrated using the same sensor scheme. The scheme reported herein is applicable to the detection of other kinds of G-rich aptamer-binding chemicals and biomolecules.

Gold nanorods-based FRET assay for ultrasensitive detection of Hg2+

A fluorescence method for detecting mercury ion in a homogeneous medium is proposed with gold nanorods (GNRs) as a fluorescence quencher on the basis of the fluorescence resonance energy transfer (FRET). Under the optimum conditions, the method exhibits a dynamic response range from 10 pM to 5 nM with a detection limit of 2.4 pM.

Metal ion sensors based on DNAzymes and related DNA molecules

Metal ion sensors are an important yet challenging field in analytical chemistry. Despite much effort, only a limited number of metal ion sensors are available for practical use because sensor design is often a trial-and-error-dependent process. DNAzyme-based sensors, in contrast, can be developed through a systematic selection that is generalizable for a wide range of metal ions. Here, we summarize recent progress in the design of DNAzyme-based fluorescent, colorimetric, and electrochemical sensors for metal ions, such as Pb(2+), Cu(2+), Hg(2+), and UO(2)(2+). In addition, we also describe metal ion sensors based on related DNA molecules, including T-T or C-C mismatches and G-quadruplexes.

A new approach to design ratiometric fluorescent probe for mercury(II) based on the Hg(2+)-promoted deprotection of thioacetals

On the basis of the protection reaction between ethanethiol and aldehyde, we designed and synthesized two new ratiometric fluorescent chemosensors, 3 and 4, by using intramolecular charge transfer (ICT) as a signaling mechanism. Upon the addition of Hg(2+) ion, both probes displayed apparent luminescence color changes, which could be observed by naked eyes under a UV lamp. Unexpectedly, both chemosensors also gave response to the addition of trace silver ions, making this kind of chemosensors as the first example of ratiometric fluorescent probe that showed dual channel fluorescence for both Hg(2+) and Ag(+). The test strips experiments suggested that 3 and 4 could serve as practical fluorescent probes for rapid detection of Hg(2+) ion.