This thesis presents a study of pH-responsive ratiometric fluorescent nanogel (NG) probes with multi-emission (blue, green, and near-infrared (NIR), respectively) and related studies in a solution or a series of hydrogels to report changes in local microenvironments. Polyacid-based NG particle probes are built containing non-radiative energy transfer (NRET) systems. The potential application of this material is to monitor and diagnose soft tissues such as those used for load support. Therefore, the research mainly involves fluorescence sensing performance, focusing on the characteristics and applications of fluorescence to monitor various responses that stimulate swelling or deswelling, and molecular release. The starting point of this study is the preparation of pH-responsive ratiometric fluorescent nanoparticles. A simple emulsion polymerisation method was used to copolymerise donor (9-phenanthryl)methylmethacrylate (Ph) and acceptor (9-anthryl)methacrylate (An) with methyl methacrylate (MMA), methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA) comonomers to form a blue-emitting nanogel NGPh/An. The NGPh/An was non-toxic and can sense pH changes in the donor and acceptor distance by stimulating swelling. Core-shell structure of NGPh/An was proposed according to the calculation of distance of NRET dyes pairs and the data of electrophoretic mobility data. Further, when the NGPh/An particles are incorporated into the three hydrogel systems (doubly crosslined nanogels, poly(acrylamide) and poly(acrylamide-Laponite)), it is possible to monitor the pH-stimulated swelling and Ca2+-stimulated deswelling in doubly crosslinked nanogel (DX NG) hydrogels, and even monitor the release of molecules. NGPh/An can also be used as a universal fluorescent probe to detect pH in a poly(acrylamide) (PAAm) hydrogel without being affected by swelling. The NGPh/An can also perform fluorescence pressure strain sensing with tough PAAm-LAP hydrogel containing poly(acrylamide) and Laponite. The NGPh/An nanoprobe system is further optimised by replacing the donor with An and using BODIPY FL amine (BDP) as a acceptor to obtain a pH-responsive green-emitted ratiometric fluorescent Poly(MMA-MAA-EGDMA-An-BDP) NGAn/BDP nanoparticles. The pathway greatly improved absolute fluorescent quantum yield and the emission is shifted 110 nm to the red. The new stable NGAn/BDP particles can be imaged at different pH values in stem cells, or excited at two different wavelengths to sense pH and divalent cations in solution. NGAn/BDP is shown to be a versatile fluorescent probe by it incorporating the probe in a range of hydrogels that exhibit ion, pH, gel degradation, temperature and tensile strain response. At the end of the project the pH-responsive ratiometric fluorescent NG probe NGCy5/Cy5.5 with deeply penetrating NIR excitation and emission is studied. The co-monomers with ethyl acrylate (EA), MAA, and 1,4-butanediol diacrylate (BDDA) was used to replace comonomers system with MMA, MAA and EGDMA. The pairs of NRET near-infrared donor and acceptor dyes used were to sulfo-cyanine5 amine (Cy5) and sulfo-cyanine5.5 amine (Cy5.5). The photoluminescence (PL) main emission peak for NGCy5/Cy5.5 is red-shifted by 150-250 nm and resides in the NIR region. In addition, I also demonstrated that the NGCy5/Cy5.5 produces a highly stable dispersion. When compared with previous temperature-responsive (poly(N-isopropylacrylamide-N,N'-methylenebisacrylamide-Cy5-Cy5.5) (pNIPAMCy5/Cy5.5) nanoprobe NGCy5/Cy5.5 can reversibly detect the pH in solution by NIR light. In addition, a range of nanoprobe concentrations are non-toxic and can be endocytosed into cells which was NIR imaged. I also captured injectable nanoprobe in chicken tissue using a NIR camera, suggesting that NGCy5/Cy5.5 has a good potential application to broad in the future.