Laser-Induced Breakdown Spectroscopy and Microscopy Study of Human Dental Tissues

*Muhammad Mustafa, Anwar Latif and Majid Jehangir*

### **Abstract**

Laser-induced breakdown spectroscopy (LIBS) analysis of human dental tissues: enamel and dentine, performed by utilizing Nd: YAG laser (=1064 =6 =50 ) to investigate threshold ablation of laser energy density. Quantitative results based on the experiment provide us with threshold ablation value of laser energy density for calcium (Ca) ablation in enamel and dentine tissues. The computed threshold laser energy density for Ca ablation in dentin tissue is 0.38 J/cm2 , which is significantly lower than the threshold in the enamel, which is 1.41 J/cm2 . Scanning electron microscopic (SEM) examination of dental tissues determines that the dentin surface contains pores, voids, and bubbles that make it easy to ablate at low laser energy density, while enamel has a closely packed smear layer structure that is difficult to ablate, requiring high energy densities. These findings are helpful in the field of laser dentistry, where lasers are widely used for dental treatment.

**Keywords:** electron microscopy, laser-induced breakdown spectroscopy, Nd: YAG laser, dental tissues, energy density

## **1. Introduction**

Lasers have been utilized as a supplementary treatment in dentistry since 1964. They have experienced significant advancements in a variety of dental applications. Nd: YAG lasers are mainly used in dentistry, including soft and hard tissue surgery, cavity preparation in tooth enamel and dentine, detection of tooth decay, prevention of tooth decay by modifying the crystalline structure of enamel, and tooth whitening [1–5].

Lasers to ablate tissues may cause cracks, fractures, fissures, irradiation roughness, surface irregularities, and the removal of smeared layers [6]. The risks of the laser can be avoided by selecting optimized laser parameters according to the chemical composition of the irradiated tissues [7, 8]. It is significant to determine the minimum value of laser energy density that is required to take out an atom of material of the same element, known as threshold ablation. To optimize the laser energy density and energy of the laser, it is required to calculate the threshold ablation of energy density for calcium (the most commonly found mineral in dental tissues) in enamel and dentine that can be precisely achieved by laser-induced breakdown spectroscopy [9, 10].

LIBS is an atomic emission spectroscopic technique. It is also known as "laserinduced plasma spectroscopy" (LIPS) because elements and atomic species are

quantified through spectrometric analysis of laser-induced plasmas. The LIBS spectroscopy technique uses a high-intensity laser capable of ablating small amounts of material, thereby creating a short-duration plasma [11]. The formed plasma contains the excited atoms, molecules, and ions that appeared in the target. As the plasma cools, the atoms, ions, and molecules lose energy due to the emission of light photons that carry certain wavelengths [12]. Thus, spectroscopic characterization of the plasma light will reveal the elements present in the target. The identification of many elemental lines, including both the wavelength and the intensity within the emission spectrum, will form a unique spectral fingerprint of the target, such as calcium [13–16].

Scanning electron microscopy (SEM) is a versatile technique to investigate surface topography, structural morphology, composition, the orientation of grains, crystallography, etc. of dental tissues by achieving its three-dimensional (3D) image with high quality and spatial resolution [17]. In SEM, micro to nanostructure analysis can be examined by focusing a finely collimated electron beam on the dental slices. Due to interaction between incident electrons and the slices, various types of signals may emit from irradiated tissues such as secondary electrons (SE), backscattered electrons (BSE), Auger electrons, continuous and characteristics X-rays, and other photons of different energies [18]. These signals are detected by suitable detectors available in SEM such as Everhart–Thornley detector (ETD) is used to detect SE [19], BSE using a retractable circular BSE detector (CBS) [20], and X-rays signal by energy dispersive spectrometer (EDS) detector [21]. 3D highly magnifying image form on the computer by connecting the signal detectors through the optical fiber.

In the present research work, we have calculated the threshold ablation value of laser energy density for calcium found in human dental tissues such as enamel and dentine by employing LIBS. To investigate the significant difference in threshold ablation for Ca in dental tissues SEM analyses are conducted to examine the surface and structural morphology of these tissues.
