Hybrid micro-/nanogels for optical sensing and intracellular imaging

Abstract

Hybrid micro-/nanogels are playing an increasing important part in a diverse range of applications, due to their tunable dimensions, large surface area, stable interior network structure, and a very short response time. We review recent advances and challenges in the developments of hybrid micro-/nanogels toward applications for optical sensing of pH, temperature, glucose, ions, and other species as well as for intracellular imaging. Due to their unique advantages, hybrid micro-/nanogels as optical probes are attracting substantial interests for continuous monitoring of chemical parameters in complex samples such as blood and bioreactor fluids, in chemical research and industry, and in food quality control. In particular, their intracellular probing ability enables the monitoring of the biochemistry and biophysics of live cells over time and space, thus contributing to the explanation of intricate biological processes and the development of novel diagnoses. Unlike most other probes, hybrid micro-/nanogels could also combine other multiple functions into a single probe. The rational design of hybrid micro-/nanogels will not only improve the probing applications as desirable, but also implement their applications in new arenas. With ongoing rapid advances in bionanotechnology, the well-designed hybrid micro-/nanogel probes will be able to provide simultaneous sensing, imaging diagnosis, and therapy toward clinical applications.

Keywords: cell imaging; hybrids; intracellular detection; microgel; nanogel; optical biosensor; stimuli-responsive polymer.

Fig. 1

Schematic diagrams showing three types of hybrid micro-/nanogel-based optical probes: (A) Type 1 where the antibody or specific targeting ligand acts as a chemical/biochemical signal receiver; (B) Type 2 where an optical moiety acts directly as the chemical/biochemical signal receiver; and (C) Type 3 where a responsive polymer gel network chain acts as the chemical/biochemical signal receiver, which will undergo a volume phase transition, change the physicochemical environment of the optical moieties, and convert the received signal into an optical signal.

Fig. 2

Schematic diagrams showing the difference between a physically and chemically cross-linked chitosan-PMAA-CdSe hybrid nanogel. Whereas the covalently cross-linked nanogels are very stable in both structure and composition upon pH variation, the hybrid nanogels based on the physical associations exhibit a significant change in the structure and composition in response to a pH increase to physiological condition. This distinction in the stability of hybrid nanogels is important for the designed multiple functions. Adapted from Reference (33). Reprinted with permission from Elsevier Publishing Group, Copyright (2010).

Fig. 3

Synthetic methods for the preparation of pure polymer micro-/nanogels via conventional and controlled radical cross-linking polymerization. Adapted from Reference (5). Reprinted with permission from RSC Publishing Group, Copyright (2010).

Fig. 4

Synthetic methods for the preparation of hybrid micro-/nanogels: (A) uploading of presynthesized optical moieties with well-defined size and shape into the preformed gels; (B) uploading of metal ions and in situ synthesis of QDs/NMNPs inside the preformed nanogels; (C) copolymerization of fluorescent monomers with other functional monomers; (D) synthesis of polymer gel shell with QD/NMNP as core template; and (E) synthesis of gel particles with preformed conjugated polymer chains semi-interpenetrated in the gel network.

Fig. 5

Schematic diagrams showing the design of the color-coded probe, a typical Type 1 probe, for detecting single biomolecules in two different binding modes. (A) Direct binding between two bioconjugated particles leading to a separation distance of d ₁. This mode of binding was used to construct rigid molecular structures (molecular rulers) for verification/validation studies of the precision in distance measurements. (B) Indirect sandwich-type binding in which two particles recognize the same target molecule at two different sites. This indirect mode of binding allows native biomolecules such as genes to be recognized and detected at the single-molecule level. Adapted from Reference (45). Reprinted with permission from National Academy of Sciences Publishing Group, Copyright (2008).

Fig. 6

(A) Model of a typical Type 2 probe, a ratiometric pH sensing nanogel, which is constructed by uploading of the pH probe dye molecules into the gel networks. (B) pH-dependent absorption of bromothymol blue in aqueous solution at pH 5.0, 7.0, and 9.0 (gray curves), and absorption and emission spectra of coumarin 6 (C6) and Nile Red (NR) in ethanol. The picture in (B) shows the green fluorescence of a mixture of C6 and NR in ethanol/water solvent (left), and the same components in the nanogel (NG) in aqueous suspension under 365 nm illumination (right). (C) Fluorescence micrographs of NRK cells incubated with a pH 7.4 buffer and loaded with the pH-responsive nanogel: (a) Fluorescence of the C6 acquired in the green channel (scale bar=20 mm); (b) fluorescence of NR acquired in the red channel; (c) overlay of (a) and (b). Adapted from Reference (60). Reprinted with permission from Wiley-VCH Publishing Group, Copyright (2010).

Fig. 7

TEM images of Au-PNIPAM core-shell hybrid nanogels of spherical (A) and flower-like shaped Au NP cores (B), and (C) their corresponding AFM images (A, top in C; B, down in C), respectively. (D) UV-Vis spectra of aqueous dispersion of the two indicated samples at the temperature below (15°C, solid lines) and above (50°C, dot lines) of PNIPAM. Adopted from Reference (95). Reprinted with permission from Wiley-VCH Publishing Group, Copyright (2008).

fig. 8

(A) Model of a ratiometric glucose sensing nanogel (Type 3 probe), which is constructed by the growth of the glucose-sensitive nanogel onto SDS-capped Ag NPs templates (10±3 nm). (B) TEM images. (C) PL response of the hybrid nanogels as a function of the glucose concentration in PBS at physiological pH and temperature. Adapted from Reference (100). Reprinted with permission from ACS Publishing Group, Copyright (2010).

Fig. 9

(A) Model of a ratiometric pH sensing nanogel (Type 3 probe), which is constructed by in situ synthesis of CdSe QDs into the pH-sensitive nanogel. (B) TEM images. (C) PL response of the hybrid nanogels as a function of the pH values. Adapted from Reference (33). Reprinted with permission from Elsevier Publishing Group, Copyright (2010).

Fig. 10

(A) Compressed z stack of confocal image of a single C6 glioma cell injected with magnesium-selective PEBBLE probes (Type 2 probe). The probes, depicted in red, reside in the cytosol and do not enter the nucleus at the center of the cell. (B) Spectra of the intercellular probes acquired on a fluorescence microscope. An aliquot of KCl was added at t=69 s. Ion channels open, resulting in an increase in intracellular free magnesium indicated by the increase in peak ratio of the probes. Adapted from Reference (116). Reprinted with permission from ACS Publishing Group, Copyright (2003).

Fig. 11

(A) Model of a ratiometric temperature sensing nanogel (Type 3 probe), which is constructed by copolymerization of a water-sensitive fluorophore DBD-AA into a temperature-sensitive PNIPAM-based nanogel. (B) Phase-contrast and fluorescence images of living COS7 cells containing the fluorescent nanogel thermometer. (C) Calibration cure and temperature resolution. Adapted from Reference (76). Reprinted with permission from ACS Publishing Group, Copyright (2009).

Fig. 12

(A) Model of a ratiometric temperature sensing nanogel (Type 3 probe), which is constructed by coating a thermo-responsive non-linear PEG-based gel shell onto the Ag-Au bimetallic NP core. (B) Typical TEM image. (C) Typical PL profiles taken at 2.5°C intervals from bottom to top except the bottom 5 curves (every 4.5°C), from 6.0 to 51.5°C. (D) Tunable temperature sensing in terms of PL intensity of the hybrid nanogels. Adapted from Reference (34). Reprinted with permission from Elsevier Publishing Group, Copyright (2010).