Photoacoustic FTIR spectroscopic study of undisturbed nacre from red abalone

In this work, photoacoustic Fourier transform infrared (PA-FTIR) spectroscopy has been utilized to study interfacial interactions of undisturbed nacre and nacre powder from red abalone shell. The spectra of both undisturbed nacre and nacre powder showed characteristic bands of aragonite and proteins. Although nacre powder and undisturbed nacre are chemically identical, PA-FTIR spectrum of undisturbed nacre is found to be significantly different from that of nacre powder. A broad and strong band is observed at around 1485 cm−1 in nacre powder. The intensity of this band is notably reduced in undisturbed nacre. This result is explained on the basis of interfacial interactions between aragonite platelets and acidic proteins. It is also observed that band at around 1788 cm−1 originates from three overlapping bands 1797, 1787 and 1778 cm−1. The band at around 1787 cm−1 is assigned to CO stretching of carboxylate groups of acidic proteins. The other two bands at 1797 and 1778 cm−1, originate from aragonite and have been assigned to combination bands, ν3 + ν4a and ν3 + ν4b, respectively. For the study of stratification in undisturbed nacre, PA-FTIR spectra have been collected in step scan mode. The variation in spectra with depth can be attributed to changes in conformation of proteins as well as interfacial interactions.