# Nanomaterials for Healthcare Biosensing Applications
## 4 Organic Nanomaterial Applications for Healthcare Biosensing
### Nanostructured Hydrogels  
Nanostructured hydrogels (NHg), or nanogels, are nanoscale 3-D cross-linked hydrophilic polymeric networks that can alter their chemical structure and volume with the help of stimuli from the external environment. Hence, they are termed stimuli-responsive smart materials. Furthermore, they have a large surface area with swelling translation, biocompatibility, and stability [241]. Based on the function of nanogels in a biosensing system, their applications can be categorised into: (i) encapsulation vehicles; (ii) multifunctional stimuli-responsive materials; and (iii) sensory membranes [242]. Nanostructured hydrogels, owing to their large surface area, are excellent for encapsulating fluorescent molecules, facilitating optical biosensing. Based on this approach, Guo's group developed a two-photon fluorescent sensor for the detection of cysteine (Cys) [243]. The \"turn-on\" probe comprised of triarylboron luminogen 4-(dimesitylboryl)-\(N\),\(N\)-diphenylaniline with \(\pi\) conjugation throughout and a methylene spacer attached to a maleimide moiety (DMDP-M). A nonogel of the triblock copolymer poloxamer 407 (POX407) was employed to encapsulate the hydrophobic DMDP-M fluorescent probes. In the presence of Cys, photoelectron transfer in the probe was hindered, generating fluorescence. The system presented a low LOD of 0.18 uM. Due to its high cell-membrane permeability and biocompatibility, the system also found successful application in NIH/3T3 cell line. Cao et al. developed a similar sensor for intracellular pH (pH) in cytosol using a polyurethane (PUR) nanogel loaded with 8-hydroxypyrene-1-carbaldehyde(HPC), a fluorescent dye [244]. The dual-emissive ratiometric detector allowed in vivo sensing in the pH range of 4-10 by undergoing a reversible and large hypsochromic shift of 100 nm in the red-green region of the visible spectrum. Multi-functional stimuli-responsive NHgs change their volume dramatically due to a change in the environmental stimuli. Exploiting this virtue, Wang et al. developed a photoluminescence sensor using NHgs for optical temperature sensing and NIR responsive drug release [245]. Temperature detection in the range of 16-50 \({}^{\circ}\)C was possible by loading a poly(\(N\)-isopropylacrylamide-co-acrylamide) (pNIPAm-Am) nanogel loaded with fluorescent carbon nanoparticles (FCNPs) (Figure 11a). Environmental temperature sensing was accomplished by manipulating the photoluminescence intensity, which was possible due to the reversible thermo-responsive transition between swelling and shrinking of the nanogel, which altered the physicochemical environment around the FCNPs. Such NHgs are called mingle structured multifunctional stimuli responsive nanogels. An alternative to this architecture is the incorporation of various kinds of nanoparticles within a nanogel shell. Li and co-workers produced such a structure using NaYF4-Yb3+-Er3 nanocrystals as the core and a thermo-responsive nanoshell comprising of a free radically polymerised \(N\)-isopropylacrylamide and \(N\)-acryllyl-\(N^{\prime}\)-rhodamine B acylhydrazine thiourea nanogel (pNIPAm-RhBHA) [246]. In the presence of 0.0-0.099 mM of Hg2+, the rhodamine moieties were transformed to 1,3,4-oxadiazoles, causing fluorescence, as illustrated in Figure 11b. The system also responded to pH, metal ions, and temperature. Kim and Li reported a similar sensor using poly(\(N\)-isopropylacrylamide) (pNIPAm) nanogel and fluorescein (Fsc) shells with vinyl functionalised carbon dot (ViCD) cores capable of detecting temperature over a wider range in comparison to the previous study [247]. The ratiometric sensor emitted reversible fluorescence from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C because of the swelling and shrinking of the hydrogel in response to temperature.