# Evaluation of pH and Optical Properties of Dual Ringe HEDP Irrigating Solution
## 4 Discussion
[31] reported a slightly higher absorption coefficient for CHX at 513 nm, with a value of \(\alpha\) = 0.42 cm\({}^{-1}\). In our study, the only observable peak absorption of \(\alpha\) = 0.12 cm\({}^{-1}\) was registered at 714 nm for CHX. This discrepancy in results could be attributed to differences in composition as well as the concentration of irrigants used in both studies. In fact, their study reported the variability in the absorption profiles of CHX at different concentration [31]. Nevertheless, the low absorption values of irrigants in the VL range deems these wavelengths unsuitable for LAI. However, lasers operating in the VL range have been used in endodontics, such as KTP lasers emitting at 532 nm that have been studied for sterilizing root canals [60; 61] and He-Ne lasers at 633 nm that were used for Laser Doppler Flowmetry [62]. Another promising aspect of laser-aided endodontics is photodynamic therapy that involves the use of a photosensitive dye (photosensitizer) and a low-intensity laser light to achieve antibacterial and tissue disinfection effects within the root canal system [63; 64]. Photosensitizers aid the rise in the absorption coefficient of water in root canals, making lasers effective in the VL region [63]. Diode lasers operating at 635 nm have shown promising efficiency in photodynamic endodontic therapy, where the photosensitizer has been combined with conventional chemo-mechanical debridement [64]. Diode lasers with wavelengths ranging from 810 to 980 nm and neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers operating at approximately 1064 nm have been extensively studied in endodontics within the NIR spectrum [65]. However, most research has centered around their application in laser irradiation rather than LAI. Notably, previous studies have suggested that water and endodontic irrigants exhibit low absorption and high transmission values in the NIR range, seemingly making these wavelengths suboptimal for LAI [31]. Despite this, both diode lasers and Nd:YAG lasers have demonstrated effectiveness in LAI. For instance, [PERSON] and [PERSON] [66] successfully generated cavitation bubbles in water using Nd:YAG lasers with very short pulses (8 ns) and lenses to tightly focus the laser beam, while [PERSON] et al. [67] employed a diode laser at 980 nm in combination with intermittent irrigation using distilled water, achieving a superior bactericidal effect compared to the conventional use of NaOCl. In another study, [PERSON] et al. [68] tested the effect of Nd:YAG (1064 nm) and diode (980 nm) lasers on EDTA agitation, showing the more positive effects of LAI than the conventional use of EDTA. Additionally, [PERSON] et al. [69] tested the efficacy of LAI with a diode laser (940 nm) in combination with 18% HEDP + NaOCl and found a highly bactericidal effect. However, their study used chemically synthesized 18% HEDP, unlike the commercially available 9% HEDP in our study. Nevertheless, the effectiveness of these wavelengths could be traced to the results of our study, where both water and endodontic irrigants presented a significant absorption peak at around 975 nm, with transmission levels of around 60%. Interestingly, the optical properties of NaOCl and HEDP + NaOCl in the NIR spectrum were found to be similar, as were those of HEDP and EDTA, in our investigation. The intriguing optical characteristics observed in our study underscore the need for dedicated research into the potential applications of these combinations in LAI within the NIR spectrum. In discussing the observed absorption peaks, it is worth considering the possibility of overtones, where higher-energy transitions involving vibrational or rotational excitations within the molecules contribute to the observed absorption features. Previous studies have reported overtones in the absorption spectra of certain molecules in the NIR range, such as water and organic compounds, typically observed as weaker absorption peaks at wavelengths approximately twice the energy of the fundamental transitions [70; 71].