Design of ratiometric emission probe with visible light excitation for determination of Ca2+ in living cells

Esterase-Activated Precipitating Strategy to Achieve Highly Specific Detection and Long-Term Imaging of Calcium Ions by Aggregation-Induced Phosphorescence Probe

Spatial and temporal monitoring of bioactive targets such as calcium ions is vitally significant for their essential roles in physiological and biochemical functions. Herein, we proposed an esterase-activated precipitating strategy to achieve highly specific identification and long-term bioimaging of calcium ions via lighting up the calcium ions by precipitation using a water-soluble aggregation-induced phosphorescence (AIP) probe. The designed probe CaP2 has an AIP behavior and can be efficiently aggregated by calcium ions through the coupling coordination of carboxylic acid and cyanide groups, which enables it to light up Ca²⁺ by precipitating-triggered phosphorescence. Four hydrophilic groups of tetraethylene glycol were introduced to endow the resulting probe CaP3 with extraordinary water solubility as well as excellent cellular penetration. Only when the probe CaP3 penetrates inside the live cells the existing esterase in cells can activate the probe to be transformed active CaP2 probe selectively binding with calcium ion in the surroundings. The probe was used to further evaluate the imaging of intracellular calcium ions in model organisms. The excellent imaging performance of CaP3 in Arabidopsis thaliana seedling roots demonstrates that CaP3 has the excellent capability of monitoring calcium ions in live-cell imaging, and furthermore CaP3 exhibits much better photostability and thereby greater potential in long-term imaging. This work established a general esterase-activated precipitating strategy to achieve specific detection and bioimaging in situ triggered by esterase in live cells, and established a water-soluble aggregation-induced phosphorescence probe with high selectivity to achieve specific sensing and long-term imaging of calcium ions in live cells.

Electrochemistry/Photoelectrochemistry-Based Immunosensing and Aptasensing of Carcinoembryonic Antigen

Recently, electrochemistry- and photoelectrochemistry-based biosensors have been regarded as powerful tools for trace monitoring of carcinoembryonic antigen (CEA) due to the fact of their intrinsic advantages (e.g., high sensitivity, excellent selectivity, small background, and low cost), which play an important role in early cancer screening and diagnosis and benefit people's increasing demands for medical and health services. Thus, this mini-review will introduce the current trends in electrochemical and photoelectrochemical biosensors for CEA assay and classify them into two main categories according to the interactions between target and biorecognition elements: immunosensors and aptasensors. Some recent illustrative examples are summarized for interested readers, accompanied by simple descriptions of the related signaling strategies, advanced materials, and detection modes. Finally, the development prospects and challenges of future electrochemical and photoelectrochemical biosensors are considered.

Multifunctional aggregation-based fluorescent probe for visualizing intracellular calcium dynamic fluctuations

Calcium ion (Ca²⁺) is an indispensable second messenger in living organisms. The impaired Ca²⁺ handling can induce many diseases. In this paper, we developed a simple and effective method to encapsulate a coumarin-based Ca²⁺ probe ((E)-2-hydroxy-N'-((7-hydroxy-2-oxo-2H-chromen-8-yl)methylene)-2-phenylacetohydrazide, CPM) into nanoparticles (NPs), and CPM NPs with blue fluorescence were obtained, whose maximum excitation and maximum emission wavelengths were characterized at 365 nm and 450 nm, respectively. The CPM NPs show significant fluorescence enhancement toward Ca²⁺ over other metal ions, with a limit of determination (LOD) of 0.04 μM. To optimize the optical property of the NPs, CPM and curcumin, which were introduced as the Förster resonance energy transfer (FRET) donor and acceptor, respectively, were co-encapsulated, and bright green CPM@Cur NPs with large stokes shift and narrow emission band width were constructed. Due to their low cytotoxicity and excellent stability, CPM NPs and CPM@Cur NPs were further successfully used to discriminate the primary aortic smooth muscle cells isolated from mice with abnormal Ca²⁺ homeostasis from their littermate controls. It is worth noting that CPM@Cur NPs exhibit stronger fluorescence signal and diminished background interference, which make them have great potential in the Ca²⁺ monitoring during biological processes. This strategy opens a new way to synthesize NPs with high brightness and has a potential application prospect in composite sensing and intracellular imaging. CPM@Cur NPs are developed and applied in biological sensing and intracellular Ca²⁺ imaging, as well as discriminating the cells with abnormal calcium homeostasis.

A novel coumarin-based fluorescent sensor for Ca2+ and sequential detection of F- and its live cell imaging

A novel fluorescent sensor CPM for relay detecting Ca²⁺ and F^- based on coumarin has conveniently synthesized and characterized. The sensor CPM showed highly fluorescence enhancement to Ca²⁺ over other metal ions, and the CPM-Ca²⁺ complex could selectively recognize F^- among other anions. The limits of detection for Ca²⁺ and F^- were 5.81 × 10^-7 M and 4.28 × 10^-7 M in aqueous solution (DMF/HEPES buffer 1:1 v/v, 10 mM, pH = 7.2), respectively. Their sensing mode had been testified by Job's plots, UV-vis titration, ¹H NMR titrations, ESI-mass, fluorescence and DFT calculations. The fluorescence imaging indicated that CPM was cell-permeable and could be used to effectively detect Ca²⁺ and F^- within living cells.

Luminescent gold nanocluster-based sensing platform for accurate H2S detection in vitro and in vivo with improved anti-interference

Gold nanoclusters (Au NCs) are promising luminescent nanomaterials due to their outstanding optical properties. However, their relatively low quantum yields and environment-dependent photoluminescence properties have limited their biological applications. To address these problems, we developed a novel strategy to prepare chitosan oligosaccharide lactate (Chi)-functionalized Au NCs (Au NCs@Chi), which exhibited emission with enhanced quantum yield and elongated emission lifetime as compared to the Au NCs, as well as exhibited environment-independent photoluminescence properties. In addition, utilizing the free amino groups of Chi onto Au NCs@Chi, we designed a FRET-based sensing platform for the detection of hydrogen sulfide (H₂S). The Au NCs and the specific H₂S-sensitive merocyanine compound were respectively employed as an energy donor and acceptor in the platform. The addition of H₂S induced changes in the emission profile and luminescence lifetime of the platform with high sensitivity and selectivity. Utilization of the platform was demonstrated to detect exogenous and endogenous H₂S in vitro and in vivo through wavelength-ratiometric and time-resolved luminescence imaging (TLI). Compared to previously reported luminescent molecules, the platform was less affected by experimental conditions and showed minimized autofluorescence interference and improved accuracy of detection.

Where is it and how much? Mapping and quantifying elements in single cells

The biological function of a chemical element in cells not only requires the determination of its intracellular quantity, but also the spatial distribution of its concentration. Different strategies can be employed to quantify and map the intracellular concentration of elements in single cells. The assessment of the intracellular elemental concentration, which is the relevant information, requires the measurement of cell volume. This challenging and demanding task requires combining different techniques allowing gathering of both morphological and compositional information on the same cell. Moreover, the need to analyse samples more similar to their natural state requires complex hardware equipment, and supplementary efforts in preparation protocols. Nevertheless, the response to the question: "where is it and how much?" is worth all these efforts. This review aims at providing an insight into the recent and most advanced techniques and strategies for quantifying and mapping chemical elements in single cells. We describe and discuss indirect detection techniques (label based) which make use of fluorescent dyes, and direct ones (label free), such as particle induced X-ray emission, proton backscattering spectrometry, scanning transmission ion spectrometry, nano-secondary ion mass spectrometry, X-ray fluorescence microscopy, complemented by X-ray imaging.

Mercury species induced frequency-shift of molecular orientational transformation based on SERS

We proposed a novel readout method based on a peculiar phenomenon in which the vibrational frequencies of a SERS-active probe (dimethyldithiocarbamic acid sodium salt, DASS) can be affected when there is mercury species.

Synthesis of an azido-tagged low affinity ratiometric calcium sensor

Changes in high localised concentrations of Ca²⁺ ions are fundamental to cell signalling. The synthesis of a dual excitation, ratiometric calcium ion sensor with a K_d of 90 μM, is described. It is tagged with an azido group for bioconjugation, and absorbs in the blue/green and emits in the red region of the visible spectrum with a large Stokes shift. The binding modulating nitro group is introduced to the BAPTA core prior to construction of a benzofuran-2-yl carboxaldehyde by an allylation–oxidation–cyclisation sequence, which is followed by condensation with an azido-tagged thiohydantoin. The thiohydantoin unit has to be protected with an acetoxymethyl (AM) caging group to allow CuAAC click reaction and incorporation of the KDEL peptide endoplasmic reticulum (ER) retention sequence.