# M13 Bacteriophage-Assisted Synergistic Optical Enhancement of Perovskite Quantum Dots
## 3 Results and Discussion
### Photoluminescence Properties of CsPbI3 PQDs with the M13 Phase in Solution Phase  
In Figure 2a, the PL spectra of pure CsPbI3 PQDs solution and its mixing with various concentrations of the M13 phage are compared under an excitation wavelength of 405 nm. The PL peaks of CsPbI3 PQDs in solution are centered at 675 nm, whereas the addition of the M13 phage induced a redshift of the PL peak. The PL enhancement was found to be concentration-dependent, as shown in Figure 2b. Increasing the concentration of the M13 phage from 0.1 mg/mL to 1 mg/mL resulted in a gradual increase in the PL intensity. We define the enhancement factor as I/I\({}_{0}\), where I and I\({}_{0}\) are the PL signals from the sample with the M13 phage and the reference sample. The highest PL enhancement of 7.7 times compared to the reference CsPbI3 PQDs is achieved with the addition of a 1 mg/mL M13 phage, whereas the lowest concentration results in an enhancement of 3 times. The concentration-dependent behavior of the M13 phage in enhancing the PL intensity of CsPbI3 PQDs can be attributed to the surface properties of the phage. At lower concentrations, the number of M13 phage particles available for binding to the PQDs is limited. As a result, fewer interactions occur, resulting in a weaker enhancement in photoluminescence (PL). On the other hand, at higher concentrations, the M13 phage particles form a dense layer on the PQDs surface, increasing the chances of interactions and leading to a stronger enhancement in photoluminescence (PL). In Figure 2c, the half-width at half-maximum (FWHM) of the PL band is almost identical to 34 nm. This suggests that the size distribution of the PQDs remains consistent across different concentrations of M13. A clear redshift in the resonance PL wavelength is noted when introducing M13 phages with different concentrations as shown in the Figure 2d. This clearly outlines the role of the M13 phage. We assume this is due to surface coverage with respect to the large number of available M13 phages. For example, a higher concentration of the M13 phage may lead to a greater number of binding sites being occupied [21], leading to a greater interaction and thus a larger redshift in the PL peak.  
Optical stability tests were conducted on PQDs and the M13 phage coupled PQDs without encapsulation to highlight the role of the M13 phage in enhancing stability. Supporting information Figure S3 shows the PL intensity degradation trends for both PQDs only and M13 phage-coupled PQDs over a 15-day period. The PL intensity of the PQDs declined rapidly within these 15 days, with a particularly steep drop in stability observed after the 6 th day. Conversely, the M13 phage-coupled PQDs exhibited a more gradual decrease in PL intensity over the same period. The PL stability deterioration percentages were recorded, with day 1 set as 100%: For PQDs only, the 6 th day was 65.2%, the 12 th day was 99.3%, and the 15 th day was 100%; for the M13 phage-coupled PQDs, the 6 th day was 20.4%, the 12 th day was 42.3%, and the 15 th day was 52.3%. These findings effectively confirm the role of the M13 phage in enhancing the lifetime stability of PQDs.