Photoacoustic measurement of refractive index of dye solutions and myoglobin for biosensing applications

Abstract

Current methods of determining the refractive index of chemicals and materials, such as ellipsometry and reflectometry, are limited by their inability to analyze highly absorbing or highly transparent materials, as well as the required prior knowledge of the sample thickness and estimated refractive index. Here, we present a method of determining the refractive index of solutions using the photoacoustic effect. We show that a photoacoustic refractometer can analyze highly absorbing dye samples to within 0.006 refractive index units of a handheld optical refractometer. Further, we use myoglobin, an early non-invasive biomarker for malignant hyperthermia, as a proof of concept that this technique is applicable for use as a medical diagnostic. Comparison of the speed, cost, simplicity, and accuracy of the techniques shows that this photoacoustic method is well-suited for optically complex systems.

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

“…These advancements would not be possible without the use of many characterization techniques, such as ellipsometry 4,6,18 , contact angle measurement, atomic force microscopy 7,11,19 , and scanning/transmission electron microscopy, which assist in the iterative improvement of these technologies by providing direct measures or indirect estimates of fundamental optical material properties. Said properties, such as the refractive index, govern how the materials interact with incident photons, which directly affects their function and their use in optical applications.…”

“…However, each of these techniques has limitations relating to resolution, sample preparation, cost, and complexity, and each generates only a subset of the data needed to fully characterize the material. That being said, a new set of techniques, known as evanescent field-based photoacoustics (EFPA) 5,6,15,18, as shown in Figure 1, has the potential to estimate material properties at the nanoscale in a consolidated set of experiments. EFPA encompasses the sub-techniques of total internal reflection photoacoustic spectroscopy (TIRPAS) 23,25,26,[33][34][35][43][44][45] , photoacoustic spectroscopy/total internal reflection photoacoustic spectroscopy refractometry (PAS/TIRPAS refractometry) /C p where α is the volume thermal expansion coefficient, v s is the speed of sound in the medium, and C p is the heat capacity at constant pressure, H 0 is the radiant exposure of the laser beam, c is the speed of sound in the excited medium, x is length, and t is time.…”

“…With such a small optical penetration depth, the evanescent field is able to interact with the environment very close to the interface of the two materials, and well below the optical and acoustic diffraction limits. The optical properties of materials or particles within this range may perturb the field or otherwise alter its generation, which interaction can be detected by a variety of methods 3,5,6,10,15,17,18,21,23,[25][26][27][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][84][85][86][87][88][89][90][91][92][93][94][95] .…”

“…This family includes, but is not limited to, total internal reflection photoacoustic spectroscopy (TIRPAS), optical tunneling photoacoustic spectroscopy (OTPAS), and surface plasmon resonance photoacoustic spectroscopy (SPRPAS). The interested reader should refer to the following references for derivations of the equations used for TIRPAS 5,6,18,23,25,26,[33][34][35][43][44][45][46][47] , PAS/TIRPAS refractometry 18 , and OTPAS 6 . In each case, the photoacoustic effect is generated through a different excitation mechanism than simple transmittance through a prism; for instance, in TIRPAS, the light is evanescently coupled through a prism/substrate/sample interface into the chromophores (which could include the sample material itself, or guest molecules within the sample), whereas in SPRPAS, the primary mode of excitation is instead through the absorption of a surface plasmon, which is a secondary E-M wave created when the energy of the evanescent field is transferred into the electron cloud of a metal layer deposited on the prism surface.…”

“…More recently, previous investigations have shown that increased sensitivity and utility are possible with modern polyvinylidene fluoride (PVDF) ultrasonic detectors and q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. Specifically, nanosecond-pulsed Nd:YAG lasers result in a 10 6 fold increase in the peak power, which enables EFPA techniques to become useful tools for evaluating the optical properties of a variety of materials and interfaces 5,6,15,18,[21][22][23][24][25][26][27][28][29][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]84,96 . Additionally, previous work has further shown the capability of such techniques to determine structural information about materials at an interface, which was previously never achievable with traditional photoacoustic spectroscopy (PAS) technologies due to their relatively large penetration depth 53,55,57,59,61,62,69,73,75,80,81 .…”