We report on the fabrication of multifunctional ratiometric probes for glucose and temperatures based on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels covalently incorporated with glucose-recognizing moieties, N-acryloyl-3-aminophenylboronic acid (APBA), fluorescence resonance energy transfer (FRET) donor dyes, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and rhodamine B-based FRET acceptors (RhBEA). P(NIPAM-APBA-NBDAE-RhBEA) microgels containing FRET pairs and APBA were synthesized via free radical emulsion copolymerization. The spatial proximity of FRET donors and acceptors within microgels can be tuned via thermo-induced microgel collapse or glucose-induced microgel swelling at appropriate pH and temperatures, leading to the facile modulation of FRET efficiencies. APBA moieties within P(NIPAM-APBA-NBDAE-RhBEA) microgels can bind with glucose at appropriate pH to form cyclic boronate moieties, which can decrease the pK a of APBA residues and increase the volume phase transition (VPT) temperature of microgels. The gradual addition of glucose into fluorescent microgel dispersions at intermediate temperatures, i.e., between microgel VPT temperatures in the absence and presence of glucose, respectively, can lead to the reswelling of initially collapsed microgels. Thus, P(NIPAM-APBA-NBDAE-RhBEA) microgels can serve as dual ratiometric fluorescent probes for glucose and temperatures by monitoring the changes in fluorescence emission intensity ratios. Moreover, P(NIPAM-APBA-NBDAE-RhBEA) microgels at pH 8 and 37 °C can serve as a ratiometric fluorescent glucose sensor with improved detection sensitivity as compared to that at 25 °C. MTT assays further revealed that thermoresponsive microgels are almost noncytotoxic up to a concentration of 1.6 g/L. These results augur well for the application of P(NIPAM-APBA-NBDAE-RhBEA) microgels for multifunctional purposes such as sensing, imaging, and triggered-release nanocarriers under in vivo conditions.

A novel sulfobetaine block copolymer, poly(N-(morpholino)ethyl methacrylate)-b-poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl)pyridinium betaine) (PMEMA-b-PSVBP), was synthesized via reversible addition-fragmentation chain transfer polymerization. In aqueous solution, PMEMA homopolymer becomes insoluble in the presence of Na2SO4 (>0.6 M), whereas PSVBP homopolymer molecularly dissolves in the presence of NaBr (>0.2 M). Thus, PMEMA-b-PSVBP diblock copolymer exhibits purely salt-responsive "schizophrenic" micellization behavior in aqueous solution, forming two types of micelles with invertible structures, that is, PMEMA-core and PSVBP-core micelles, depending on the concentrations and types of added salts (Scheme 1). The equilibrium structures of these two types of micelles were characterized via a combination of 1H NMR and laser light scattering (LLS). We further investigated the kinetics of salt-induced formation/dissociation of PMEMA-core and PSVBP-core micelles and the structural inversion between them employing the stopped-flow light scattering technique. In the presence of 0.5 M NaBr, the addition of Na2SO4 (>0.6 M) induces the formation of PMEMA-core micelles stabilized with well-solvated PSVBP coronas. Dilution-induced dissociation of PMEMA-core micelles into unimers occurs within the dead time of the stopped-flow apparatus (approximately 2-3 ms) when the final Na2SO4 concentration drops below 0.3 M, while salt-induced breakup of PSVBP-core micelles is considerably slower. The structural inversion from PMEMA-core to PSVBP-core micelles proceeds first with the dissociation of PMEMA-core micelles into unimers, followed by the formation of PSVBP-core micelles. On the other hand, structural inversion from PSVBP-core to PMEMA-core micelles exhibits different kinetic sequences. Immediately after the salt jump, PMEMA corona chains are rendered insoluble, and unstable PSVBP-core micelles undergo intermicellar fusion; this is accompanied and/or followed by the solvation of PSVBP cores and structural inversion into colloidally stable PMEMA-core micelles.

It is known that the zwitterionic diblock copolymer, poly(4-vinylbenzoic acid)-b-poly(N-(morpholino)ethyl methacrylate) (VBA-b-MEMA), exhibits interesting "schizophrenic" micellization behavior (see: Liu, S.; Armes, S. P. Langmuir 2003, 19, 4432-4438). The kinetics of the pH-induced formation and dissociation of VBA-core micelles, the salt-induced formation and dilution-induced dissociation of MEMAcore micelles at pH 10, and the pH-induced micellar inversion between VBA-and MEMA-core micelles in the presence of 0.8 M Na 2 SO 4 were studied in detail using stopped-flow apparatus equipped with a light scattering detector. A pH jump from 12 to 2 in the absence of salt leads to the formation of VBA-core micelles; upon a pH jump from 2 to 12, the breakup of VBA-core micelles into unimers occurs within the dead time of the stopped-flow apparatus (∼ 2-3 ms). At pH 10, addition of Na 2 SO 4 (> 0.6 M) induces the formation of MEMAcore micelles. Compared to the pH-induced formation and dissociation of VBA-core micelles, the salt-induced formation of MEMA-core micelles is faster, while the dilution-induced dissociation of MEMA-core micelles into unimers is considerably slower. This partially reflects the block length asymmetry of this VBA-b-MEMA copolymer and also the fact that the MEMA-core micelles are denser and larger than the VBA-core micelles. The structural inversion from VBA-core micelles to MEMA-core micelles upon a pH jump from 2 to 12 in the presence of 0.8 M Na 2 SO 4 proceeds first with the fusion of VBA-core micelles into lose aggregates due to the insolubility of MEMA shell immediately after pH jump, then the dissociation of VBA-core micelles into unimers and partial disintegration of initially formed loose aggregates, which is followed and/or accompanied by the reaggregation of unimer chains into MEMA core-micelles. The structural inversion from MEMA-core micelles to VBA-core micelles on jumping from pH 12 to 2 in the presence of 0.8 M Na 2 SO 4 exhibits different kinetics. The scattering intensities decrease monotonically with time and then stabilize out. All the relaxation curves at different copolymer concentrations can be well-fitted using a single-exponential function and the characteristic relaxation time for the structural inversion of the micelles (τ i ) is ∼0.3 s, which slightly decreases with increasing copolymer concentrations. We tentatively propose that the structural inversion from MEMA-core to VBA-core micelles proceeds first with the splitting of large MEMA-core micelles into small VBA-core micelles, followed and/or accompanied by the redistribution of unimer chains between appearing small VBA-core micelles.

We report on the fabrication of fluorescent pH-sensing organic/inorganic hybrid mesoporous silica nanoparticles (MSN) capable of tunable redox-responsive release of embedded guest molecules. The reversible addition-fragmentation chain transfer (RAFT) copolymerization of N-(acryloxy)succinimide (NAS), oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA), and 1,8-naphthalimide-based pH-sensing monomer (NaphMA) at the surface of MSN led to fluorescent organic/inorganic hybrid MSN. The obtained hybrid MSN exhibits excellent water dispersibility and acts as sensitive fluorescent pH probes in the range pH 4-8 due to the presence of NaphMA moieties. After loading with rhodamine B (RhB) as a model drug molecule, P(NAS-co-OEGMA-co- NaphMA) brushes at the surface of hybrid MSN were cross-linked with cystamine to block nanopore entrances for the effective retention of guest molecules. Taking advantage of disulfide-containing cross-linkers, the release rate of RhB can be easily adjusted by adding varying concentrations of dithiothreitol (DTT), which can cleave the disulfide linkage to open blocked nanopores. The increase of DTT concentration from 0 to 20 mM led to 20-30 times enhancement of RhB release rate. The reported multifunctional hybrid MSN augurs well for applications in controlled-release nanocarriers, cell and tissue imaging, and clinical diagnosis.

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We report on the fabrication of Cu2+-sensing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels labeled with metal-chelating acceptor and fluorescent reporter moieties. Cu2+ detection sensitivity can be considerably enhanced via thermo-induced collapse of the sensing matrix, which can easily optimize the relative spatial distribution of Cu2+-binding sites and fluorescence readout functionalities. A novel picolinamine-containing monomer with Cu2+-binding capability, N-(2-(2-oxo-2-(pyridine 2-yl-methylamino)ethylamino)ethyl)acrylamide (PyAM, 3), was synthesized at first. Nearly monodisperse Cu2+-sensing microgels were prepared via emulsion polymerization of N-isopropylacrylamide (NIPAM) in the presence of a nonionic surfactant, N,N'-Methylene-bis(acrylamide) (BIS), PyAM (3), and fluorescent dansylaminoethyl- acrylamide (DAEAM, 5) monomers at around neutral pH and 70 degrees C. At 20 degrees C, as-synthesized microgels in their swollen state can selectively bind Cu2+ over other metal ions (Hg2+, Mg2+, Zn2+, Pb2+, Ag+, and Al3+), leading to prominent quenching of fluorescence emission intensity. Above the volume phase transition temperature, P(NIPAM-co-PyAM-co-DAEAM) microgels exhibit increased fluorescence intensity. It was observed that Cu2+ detection sensitivity can be dramatically enhanced via thermo-induced microgel collapse at elevated temperatures. At a microgel concentration of 3.0x10(-6) g/mL, the detection limit drastically improved from approximately 46 nM at 20 degrees C to approximately 8 nM at 45 degrees C. The underlying mechanism for this novel type of sensor with thermotunable detection sensitivity was tentatively proposed.

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We report on the fabrication of ratiometric fluorescent K(+) sensors based on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels covalently incorporated with K(+)-recognizing 4-acrylamidobenzo-18-crown-6 residues (B18C6Am), fluorescence resonance energy transfer (FRET) donor dyes, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and rhodamine-B-based FRET acceptors (RhBEA) by utilizing K(+)-induced changes in microgel volume phase transition (VPT) temperatures. P(NIPAM-B18C6Am-NBDAE-RhBEA) microgels were synthesized via the free radical emulsion copolymerization technique. The spatial proximity between FRET pairs (NBDAE and RhBEA dyes) within microgels can be tuned via thermoinduced collapse and swelling of thermoresponsive microgels above and below VPT temperatures, leading to the facile modulation of FRET efficiencies. B18C6Am moieties within P(NIPAM-B18C6Am-NBDAE-RhBEA) microgels can preferentially capture K(+) via the formation of 1:1 molecular recognition complexes, resulting in the enhancement of microgel hydrophilicity and elevated VPT temperatures. Thus, the gradual addition of K(+) into microgel dispersions at intermediate temperatures, i.e., between VPT temperatures of P(NIPAM-B18C6Am-NBDAE-RhBEA) microgels in the absence and presence of K(+) ions, respectively, can directly lead to the reswelling of initially collapsed microgels. This process can be monitored by changes in fluorescence intensity ratios, i.e., FRET efficiencies. The presence of FRET pairs within P(NIPAM-B18C6Am-NBDAE-RhBEA) microgels allows for the facile in situ monitoring of thermoinduced and K(+)-induced VPTs of dually responsive microgels. The response time for fluorescent K(+)-sensing was further investigated via the stopped-flow technique, which reveals that the process completes within ∼4 s. This work represents the first report of thermoresponsive microgel-based ratiometric fluorescent sensors for both K(+) ions and temperatures.