Generation of near-UV Absorption Band at Extremely Alkaline pH in Colloidal Coffee

Coffee and its primary ingredient, caffeine, have been used as solar absorbers and also to increase the thermal stability and efficiency of perovskite solar cells. In this work, we have reported the generation of an optical absorption band in the near-UV region in colloidal coffee solution at extremely alkaline pH values. This generation of absorption band could be explained by the orientation of various induced dipole-dipole interactions arising from varying pH. The said generation is due to narrowing of direct as well as indirect bandgaps from 4 eV→2.8 eV & 3.4 eV→2.5 eV, respectively. Additionally, we have estimated the changes in optical energy storage efficiency, inferring it to be highest for pH 11 having the highest intensity of the generated absorption band (λabs≈360 nm). With these observations and further deductions, the work reported in this paper would be of immense interest to the researchers working in the field of development of novel UV absorbers and also in the development of chemical pH sensors. Keyword: Chemical and biological sensors, Caffeine, Colloidal coffee, Optical absorption, pH sensor, Solute-solvent interactions, UV absorber.


INTROdUCTION
Coffee is one of the most consumed beverage around the world 1 . Its primary constituent is caffeine, which stimulates the central nervous system 2 and is the world's most widely consumed psychoactive substance that is legally permissible 3 . Recently, coffee and its ingredient caffeine have garnered attention of researchers working in the field of photovoltaics. Colloidal solution of coffee has been shown to be useful as a material for direct solar absorption 4 whereas caffeine has been used to improve the performance and thermal stability of perovskite solar cells 5 . Recently, an alternative way to sense extremely alkaline pH using the shape of hydrophobe-water interfaces was reported 6 .
Earlier, researchers have studied the effects of pH variation on the absorption spectra of various organic compounds [7][8][9] . In this paper, we have reported the sensing of extremely alkaline pH (11 ≤ pH ≤ 13) aided by generation of an optical absorption peak (λ abs ≈360 nm) in the near-UV region when the pH of the colloidal coffee solution was within the stipulated range. However when the pH of colloidal solution was less than or equal to 10, the aforementioned absorption peak was not found. Henceforth it can be inferred that colloidal coffee has an indirect optical bandgap, which can be further modified to a lower value by the generation of a new absorption band by changing the pH of colloidal coffee. In continuation to the earlier discussion, since coffee and its ingredient caffeine have now gained attention of researchers working in the field of solar photovoltaics, this work would be of immense interest to the same group especially to those working on the bandgap engineering and further development of novel UV absorbers.

MATERIALS ANd METHOd
Commercially available instant coffee Nescafe Classic was used in this work. Estimated caffeine content in such products is ≈ 26 mg/g of instant coffee 10 . A stock colloidal solution of 1 mg/mL was prepared by mixing instant coffee with Milli-Q water. 0.1 mL of the colloidal coffee was taken from the prepared stock solution and mixed with 2.6 mL of Milli-Q water and 0.3 mL of aqueous solutions of varying pH (pH: 0-14) prepared by serial dilution. For the acidic phase, 35% HCl (Emplura Merck, 1.93401.0521) was taken as stock solution. Normality of the stock HCl solution was calculated to be ≈ 11.33. Since HCl is a monoprotic acid, the molarity would be same as that of normality i.e. ≈ 11.33 M. Suitable dilutions of the same were done to obtain solutions in the range of pH 0 (1 M) till pH 6 (10-6 M). Milli-Q water was taken as a neutral standard. For the alkaline phase, 1 M NaOH solution was prepared as stock using NaOH pellets (Himedia, MB095). Serial dilutions of NaOH stock were done to obtain solutions in the range pH 8 (10 -6 M) till pH 13 (10 -1 M). Optical absorbance and transmittance measurements were done using U-2910 Hitachi spectrophotometer.

RESULTS ANd dISCUSSION
The phenomenon of generation of optical absorption band in colloidal coffee can be depicted schematically with the aid of an E-k diagram, as shown in Figure 1.   Absorption coefficient (a) has been calculated using the relationship:a 12 . Here, l is the path length traversed by light during absorption/transmission measurements (taken to be 1 cm), T is the measured transmittance and R is reflectance. Since the measurements in this work were carried out in liquid phase, reflectance has been considered to be negligible i.e. R = 0. Direct and indirect bandgaps were calculated by Tauc plot analysis i.e. by plotting (ahn) 2 & (ahn) 0.5 vs. hn (energy of the measured electromagnetic spectrum, in eV), respectively 13 . The calculated direct and indirect bandgaps from the Tauc plots are shown in Figures 3 & 4, as follows: Generation of the said optical absorption band can be attributed to the solute-solvent interactions in colloidal coffee. These solute-solvent interactions can be described by the following analytical relationships 14 , as discussed in the following equations, 1-3. (1) Where ∆n ind = n o -n s is the change in wavenumber due to induced dipole moments accounting for solute-solvent interactions, n o is the wavenumber for isolated molecule in vapor phase (in this case, caffeine) & n s is the wavenumber of caffeine molecule in solution. n&∈ are solvent refractive index & dielectric constant, respectively. a is the molecular radius of caffeine. μ g &μ e are induced dipole moments in the ground & excited states and θ is the angle between μ g &μ e vectors. There can be different types of dipole-dipole interactions that might impact the generation of the absorption band, which can be classified into orientation interactions & dispersion force mediated interactions, respectively. As the name suggests, the orientation interactions represent the interactions of dipoles based on their orientations, as given by the relationship shown in equation 2. (2) where and m e is the mass of an electron. The orientation interaction mediated wavenumber shift is affected by the increasing polarity of the solvent. The dispersion force mediated interactions happen because of the electronic structure deformations of solute and solvent leading to the generation of small temporary dipole moments. Analytically, these interactions can be represented by the relationship shown in equation 3. (3) Where a g & a e are polarizabilities of caffeine molecule in ground and excited states after interaction with the solvent medium. I&I' represent the ionizing potentials in the ground and excited states, respectively. The dispersion force mediated interactions have a mere additive effect that is primarily impacted by the refractive index, n, of the solvent. On the basis of discussions made in context of equations 2 & 3, it can be deduced that the main reason of generation of the optical absorption band at λ abs ≈360 nm is because of the orientation interactions between caffeine and aqueous solvents at pH 11-13. Since these solvents are extremely polar in nature, the electronic structure deformation of the caffeine-base complex formed from the solutesolvent interaction would cause the temporary dipole generation, eventually leading to the generation of the said optical absorption band.
We have also estimated the absorptive optical energy storage efficiency by first normalizing the transmittance values, T=e -a λ l ) (a λ : wavelengthdependent absorption coefficient), from 0 to 1. After normalizing, the transmittance values have been subsequently put into the following relationship that yields the energy storage efficiency [4] shown in equation 2: Where I λ is the wavelength-dependent intensity of incident radiation and it has been assumed that there is no scattering of light from the colloidal sample. Since the measurements were done using a conventional spectrophotometer in laboratory settings, I λ can be assumed to be constant, equalling to the intensity of the lamp of spectrophotometer. The integrals given in equation 4 were solved numerically using composite trapezoidal rule 15 . The stored energy fraction, thus calculated, has been depicted in Fig. 5, with an evident increase in optical energy storage efficiency for the generated absorption band at 360 nm in the pH range 11-13.

CONCLUSION
In this work, we have reported the sensing of extremely alkaline pH with the aid of colloidal coffee solution and also observed the generation of an optical absorption band with an indirect bandgap of ~ 2.5 eV. As discussed earlier in the paper, coffee and its primary ingredient caffeine have found applications as solar absorbers as well as in increasing the stability & efficiency of perovskite solar cells. The results reported in this paper could be used to develop ultraviolet light absorbers with increased capacity in optically absorbed energy storage in the near-UV range of the electromagnetic spectrum, actuated by modulating pH values of the colloidal coffee solution. This work also paves way towards the development of novel chemical sensors for extreme alkaline pH based on the spectrophotometric detection.