Nitroxide radicals, such as 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and its derivatives, have recently been used as contrast agents for magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). However, their rapid one-electron bioreduction to diamagnetic N-hydroxy species when administered intravenously has limited their use in in vivo applications. In this article, a new approach of silica coating for carrying stable radicals was proposed. A 4-carboxyl-TEMPO nitroxide radical was covalently linked with 3-aminopropyl-trimethoxysilane to produce a silanizing TEMPO radical. Utilizing a facile reaction based on the copolymerization of silanizing TEMPO radicals with tetraethyl orthosilicate in reverse microemulsion, a TEMPO radicals doped SiO2 nanostructure was synthesized and coated on the surface of NaYF4:Yb,Er/NaYF4 upconversion nanoparticles (UCNPs) to generate a novel multifunctional nanoprobe, PEGylated UCNP@TEMPO@SiO2 for upconversion luminescence (UCL) and magnetic resonance dual-modality imaging. The electron spin resonance (ESR) signals generated by the TEMPO@SiO2 show an enhanced reduction resistance property for a period of time of up to 1 h, even in the presence of 5 mM ascorbic acid. The longitudinal relaxivity of PEGylated UCNPs@TEMPO@SiO2 nanocomposites is about 10 times stronger than that for free TEMPO radicals. The core–shell NaYF4:Yb,Er/NaYF4 UCNPs synthesized by this modified user-friendly one-pot solvothermal strategy show a significant enhancement of UCL emission of up to 60 times more than the core NaYF4:Yb,Er. Furthermore, the PEGylated UCNP@TEMPO@SiO2 nanocomposites were further used as multifunctional nanoprobes to explore their performance in the UCL imaging of living cells and T1-weighted MRI in vitro and in vivo.

Blue fluorescent carbon dots (CDs) were synthesized by a microwave assisted one-step procedure without surface passivation. The intermolecular interactions between the CDs and nitroxide radicals in aqueous and aprotic solution were investigated. The fluorescence of the CDs was found to be efficiently quenched by the paramagnetic nitroxide radical. A SOMO (singly occupied molecular orbital) facilitated electron transfer was proposed to account for the quenching of the fluorescence of the CDs in the CD@TEMPO conjugate, formed via electrostatic interactions between the negatively charged CDs and the cationic 4-amino-2,2,6,6-tetramethylpiperidine-N-oxide free radical (4-AT). Covalently spin-labeled CDs by the paramagnetic nitroxide radical yield a weakly fluorescent spin-labeled conjugate (CD–TEMPO), which exhibits sensitive fluorescence and electron spin resonance (ESR) bimodal response towards ascorbic acid (AA) at the μM level. The fluorescence intensity of the spin-labeled CDs was also found to be sensitive to paramagnetic factors and thus establishes its promising potential for constructing a bimodal responsive sensor with off–on fluorescence and on–off ESR signaling for the detection of antioxidants and carbon-centered radicals.

Herein we propose a novel method for ultrasensitive detection of phenolic compounds. This method was developed based on a spin-labeled terbium complex Tb3+/cs124-DTPA-TEMPO (1). This spin-labeled terbium complex is a weakly luminescent compound and shows strong off-on luminescent response to phenolic compounds in the presence of horseradish peroxidase (HRP), glutathione (GSH) and hydrogen peroxide. The analyte recognition and signaling mechanism are discussed and the factors affecting the off-on luminescence have been explored. Detection limits of 1.1 nM for phenol, 1.1 nM for resorcine, 0.6 nM for m-cresol, 3 nM for p-cresol, and 0.5 nM for 2,4-dichlorophenol were obtained, respectively. The practicability of the proposed method has been tested in detection of the concentration of spiked nearshore seawaters, and recoveries of 77.4–80.4% with relative standard deviations (RSDs) of 1.0–2.2% were obtained.

A new water-soluble anionic conjugated polyelectrolyte with a nitroxide radical covalently linked to the sulfonated poly(phenylene ethynylene) backbone (PPE-SO3) is reported. This radical-functionalized PPE-SO3 (RF-PPE-SO3) demonstrates fluorescence and electron spin resonance (ESR) bimodal signaling function and shows sensitive and selective response to antioxidants.

We developed a novel spin-labeled terbium complex Tb3+/cs124-DTPA-TEMPO (1) by covalently labeling a nitroxide radical on the terbium complex for monitoring free radicals of various areas. This lanthanide complex probe shows a high EPR signal which resulted from the nitroxide radical moiety, and is weakly luminescent which resulted from the intramolecular quenching effect of the nitroxide radical on sensitised terbium luminescence. The intensity of both the EPR and luminescence can be modulated by eliminating the paramagnetism of the nitroxide radical through recognition of a carbon-centered radical analyte and thus gives a quantification of the analyte. We have preliminarily applied this probe in the luminescent detection of model carbon-centered radicals and hydroxyl radicals (·OH). This probe is water-soluble and contains lanthanide-luminescence properties, favorable for the time-resolved luminescence technique. The investigation of the intramolecular quenching process has showed that the labeled nitroxide radical quenches multiple excited states of the terbium complex, resulting in highly efficient quenching of terbium luminescence. This probe is the first example of intramolecular modulation of lanthanide luminescence by a nitroxide radical.

A new fluorescence sensor for the highly selective detection of Cu2+ ion with a detection limit of 3.6 nM based on the aggregation-induced fluorescence quenching of the highly fluorescent glutathione-capped gold nanoparticles is reported.

Positively charged CdTe-QDs capped with cysteamine (CA-CdTe-QDs) and negatively charged AuNPs capped with 11-mercaptoundecanoic acid (MUA-AuNPs) have been prepared. They are water-soluble and biocompatible. An assay for the determination of Pb2+ has been proposed based on the modulation in FRET efficiency between QDs and AuNPs in the presence of Pb2+, which inhibits the interaction of the QD-AuNP assembly. This method is easy to operate and with remarkably high sensitivity. Under the optimum conditions, the response is linearly proportional to the concentration of Pb2+ in the range 0.22–4.51 ppm, and the detection limit is found to be 30 ppb of Pb2+ due to the superior fluorescence properties of QDs. The mechanism of this strategy is also discussed.

A new water-soluble anionic conjugated polyelectrolyte with a nitroxide radical covalently linked to the sulfonated poly(phenylene ethynylene) backbone (PPE-SO3) is reported. This radical-functionalized PPE-SO3 (RF-PPE-SO3) demonstrates fluorescence and electron spin resonance (ESR) bimodal signaling function and shows sensitive and selective response to antioxidants.

A new fluorescence sensor for the highly selective detection of Cu2+ ion with a detection limit of 3.6 nM based on the aggregation-induced fluorescence quenching of the highly fluorescent glutathione-capped gold nanoparticles is reported.