# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 3 Stimuli Dependent Techniques
### Physical Stimuli-Based Self-Assembly of Nanomaterials
#### 3.2.4 Temperature-Induced Self-Assembly
While the metal-dependent plasmonics have demonstrated several applications in the broad arena of photonics and biosensing, recently there is a growing interest towards engineering nanostructures with metal-dielectric hybrid interfaces to realize less lossy photo-plasmonic interfaces [123; 124]. In this context, the cryosorets have been synthesized using high refractive index TiO\({}_{2}\) NRs where the NRs are incorporated into the solution of AgNPs prior to exposure to the thermal gradient. The TEM images of obtained AgTiO\({}_{2}\) cryosorets are shown in Figure 9b-e, with the HRTEM presented in Figure 9f, displaying the characteristic lattice fringes of Ag and TiO\({}_{2}\). The performance of the synthesized cryosorets was tested for enhancing the fluorescence signal intensity over the surface plasmon-coupled emission (SPCE) platform. The experiments were carried out in three configurations: (i) spacer, (ii) cavity, and (iii) ext. cavity nanointerfaces. In the spacer nanointerface, the cryosoret functions as a spacer between the metal thin film and the radiating dipoles. In the cavity nanointerface, the radiating dipole is sandwiched between the cryosoret and the thin metal film where the radiating dipole is in the cavity hotspot environment. The cavity is extended to a known distance in the ext. cavity nanointerface, where the cryosoret and the radiating dipoles are separated with a particular distance to avoid surface-induced quenching [38]. The experimental results of the SPCE enhancements obtained for studying these materials in spacer, cavity, and ext. cavity nanointerfaces are presented in Figure 9. The SPCE enhancements obtained with the use of different types of cryosorets are presented in Figure 9g. It is seen that the SPCE increased for all the samples as compared to the blank, indicating the effective nano-engineering of the SPCE interface assists in boosting the fluorescence signal intensity. While the plasmonic AgNPs yielded \(\sim\)60-fold enhancements, the TiO\({}_{2}\) spheres and rods presented \(\sim\)50-fold and \(\sim\)100-fold SPCE enhancements. Moreover, while such materials were utilized for the synthesis of cryosorets, \(>\)1000-fold enhancements in the fluorescence signal were observed. The SPCE, PS, p, and spectra along with the directionality data is presented in Figure 9h,i, respectively, for the sample yielding the highest SPCE enhancements in Figure 9g. Figure 10 presents an executive summary of the key research highlights of CSNE technology. From Figure 10a-c we see that the number of NPs forming the assembly increases with the decrease in the temperature (\(-18\,^{\circ}\)C to \(-80\,^{\circ}\)C to \(-150\,^{\circ}\)C to \(-196\,^{\circ}\)C) for plasmonic AgNPs and AuNPs. This indicates that NP assemblies can be obtained at reduced time intervals by using lower temperatures. Further performance of the synthesized AgTiO\({}_{2}\) (R) CS was tested for sensing RhB at trace concentrations, where RhB serves as an SPCE reporter molecule. From Figure 10d-g we observe that a linear relationship is observed for the RhB sample sensing with zeptomolar limit of detection. This high sensitivity is attributed to the metal-dielectric hybrid coupling of the AgTiO\({}_{2}\) (R) CS with the surface plasmon polaritons (SPPs) of the metallic thin film. Further, the obtained emission was captured using a smartphone camera and analyzed using a simple Color Grab app to obtain the luminosity values. It is also seen that there is an excellent correlation between the SPCE enhancements and the luminosity values, thereby emphasizing that the sensing can be effectively performed using a cost-effective smartphone-based detector platform. Furthermore, it is important to comment upon the effectiveness of the fourth generation of plasmonic hotspots that are successfully demonstrated using the CSNE protocol (Figure 10h). The plasmonic NP with an ability to confine the incident EM radiation to local regimes on account of the LSPR effect comprises the first generation of plasmonic hotspots.