4 ± 0.6 mV to 8.69 ± 1.3 mV after adding 30 μL NaOH (Table 1). Furthermore, to verify the influence of free MUA in the solution towards the LSPR shift, we found that there was a consistence LSPR shift trend between washed and unwashed GNR-MUA samples. These results demonstrated that the observation of pH-dependent
LSPR shift was apparently related to the changes Proteases inhibitor in the charge of the carboxylic acid groups of MUA bond on GNR instead of free carboxylic groups of MUA (Additional file 1: Figure S3). Figure 4 Reversibility of LSPR shift from GNP, GNP-UDT, and GNP-MUA between pH 2.60 and 11.75. Based on the above observation, subsequent experimental efforts have focused on the reversibility of the system. The titration procedure was repeated several times, going up and down on the pH scale. The LSPR of as-synthesized GNRs and GNR-UDT remains unchanged after the addition of 30 μL NaOH/HNO3 (Figure 4). This result is in good agreement with the result presented above that the LSPR of
as-synthesized GNR and uncharged GNR-UDT was definitely not influenced by pH fluctuation. In comparison, the LSPR shift of GNR-MUA as a function of pH was found to be reversible between pH 11.75 and pH 2.60. Hence, these results indicate that the reversible change to the plasmon of these GNR tethered with MUA shows pH dependence, and this phenomenon demonstrates the utility of our pH nanosensor in a specific range of pH conditions. The LSPR shift Silmitasertib concentration of GNR-MUA is 10.5 nm (821.5 to 832 nm) within the pH range of 6.41 to 8.88 (Figure 5). The S-shaped curve has a linear response range between
pH 6.41 and 7.83. The slope of 5.11 indicated that there was a 5-nm shift of LSPR for each unit change of pH value. This pH-sensing range suggests potential application for pH determination in living-cell organelles such as endosomes and lysosomes, especially for the detection of specific tumor cells for which the cellular pH is within a Carnitine palmitoyltransferase II range between 6.40 and 6.90 [17]. Figure 5 LSPR shift of GNR-MUA ligands as a function of pH in solution. It is well established that the peak wavelength, λ max, of the LSPR is dependent upon the size, shape, and distance between nanoparticles, as well as its dielectric properties and the changes in the effective refractive index (RI) of local surrounding environment including substrate, solvent, and adsorbates [38]. The dependence of LSPR or Fano resonance peak maximum [39] on RI which changes near the metal surface has been utilized in many plasmonic sensing applications. According to the modified equation of the LSPR wavelength shift Δλ max = mΔn(t/l) by Malinsky et al.