Photophysical and Nonlinear Optical Properties of Carmine Encapsulation Compared with Ionic Solutions and Environmental Polarity
Study on the Photophysical and Nonlinear Optical Properties of Carmine Encapsulation
Background Introduction
Nonlinear optical (NLO) materials have received widespread attention in recent years in fields such as laser technology, medicine, and biomedical imaging. Due to their unique optical properties, such as optical switching, optical limiting, and optical processing, these materials hold significant importance in photonics. Organic dye molecules, with their π-electron delocalization characteristics, exhibit notable nonlinear optical responses, making them a research hotspot. Carmine, a natural dye extracted from insects, is widely used in the food industry and art due to its excellent photophysical properties and stability. However, its photophysical behavior and nonlinear optical properties in different environments have not been fully studied.
To further explore the potential of Carmine in nonlinear optics, researchers have attempted to enhance its performance through microemulsion encapsulation technology. This technique encapsulates dye molecules within nanoscale water droplets, thereby altering their polarity and optical properties. Additionally, the study investigated the effects of various additives (such as CTAB, NaCl, and NaOH) on the optical properties of Carmine, aiming to provide theoretical support for developing new photosensitizers and optical devices.
Research Source
This paper was co-authored by Tina Moharer Ahmadi and Soheil Sharifi, both from the Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Iran. It was published in the journal Optical and Quantum Electronics in 2025, with the DOI: 10.1007/s11082-025-08053-y.
Research Details
a) Research Process
1. Material Preparation
The researchers used Carmine, AOT (sodium bis(2-ethylhexyl)sulfosuccinate), ethanol, CTAB (cetyltrimethylammonium bromide), NaCl, NaOH, and n-heptane provided by Sigma-Aldrich to prepare various solutions. Specifically, these included: - Aqueous Solutions: Carmine concentrations were 0.03 mM and 0.013 mM, with varying proportions of ethanol (10%-90%) and different concentrations of NaCl, CTAB, and NaOH. - Encapsulated Samples (CR-Cap): By adjusting the molar ratio of water to AOT (X=[H2O]/[AOT]) and the droplet mass fraction (Mf=(MH2O+MSur+MCr)/Mtotal), encapsulated samples with X=10, Mf=0.07, and Carmine concentration of 0.013 mM were prepared.
2. Experimental Methods
The experiment used a Z-scan instrument to measure the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) of Carmine. A continuous-wave laser with a wavelength of 532 nm and power of 80 mW was used, with a focusing focal length of 5.0 cm and a beam radius of 14 µm. The samples were placed in 2 mm thick sample cells, and a stepper motor was used to control the position of the laser passing through the samples.
Additionally, the researchers used a UV-1650 PC spectrophotometer and an FP-6200 spectrofluorometer to measure the absorbance and Rayleigh scattering (RS) of the samples. The particle size distribution of the encapsulated samples was determined using a Malvern particle size analyzer.
3. Data Analysis
The researchers utilized quantum perturbation theory to calculate the ground and excited state dipole moments of Carmine in different solutions and analyzed the changes in fluorescence spectra through the Lippert-Mataga equation. Furthermore, the relative integrated ratio (RIR), defined as the ratio of the integrated intensity of the scattering curve to the area of absorbance, was calculated to quantify the relationship between scattering and nonlinear absorption.
b) Main Results
1. Characteristics of Encapsulated Samples
The encapsulated sample (CR-Cap) had a particle size of 6.4 ± 1 nm and a PDI of approximately 0.5, indicating uniform nanoscale dispersion. Carmine dissolved in the aqueous phase was successfully encapsulated within nanoscale water droplets in the oil phase (n-heptane).
2. Photophysical Properties
- Absorbance and Optical Bandgap: The absorption peaks of Carmine in aqueous solutions were located at 300-400 nm (π→π* transition) and 500-600 nm (n→π* transition). The optical bandgap was 2.1 eV, unaffected by solvent polarity or additives.
- Fluorescence Spectra: Carmine exhibited fluorescence in the range of 320-550 nm, with the main peak position shifting to shorter wavelengths as the ethanol concentration increased. The fluorescence spectrum of the encapsulated sample showed significant changes, indicating alterations in its molecular environment.
3. Nonlinear Optical Properties
- Nonlinear Absorption Coefficient (β): The β value of the encapsulated sample was 44.6 × 10⁻⁵ cm/W, nine times higher than that in the aqueous solution. The β value increased with increasing ethanol concentration, NaOH, and CTAB concentrations, while it decreased with increasing NaCl concentration.
- Nonlinear Refractive Index (n2): The n2 value of the encapsulated sample was 12.6 × 10⁻⁹ m²/W, 4.4 times higher than that in the aqueous solution. This was attributed to the low thermal conductivity of n-heptane (0.14 W·m⁻¹K⁻¹) and changes in the dipole moment of the Carmine molecule.
4. Relationship Between Rayleigh Scattering and Nonlinear Absorption
The study found that samples with low Rayleigh scattering typically exhibited lower nonlinear absorption coefficients. The encapsulated sample, due to reduced polarity and molecular aggregation effects, demonstrated higher β values and stronger Rayleigh scattering.
c) Conclusions and Significance
This study revealed the significant enhancement of the nonlinear optical properties of Carmine through encapsulation technology. The nonlinear absorption coefficient and nonlinear refractive index of the encapsulated sample increased by nine times and 4.4 times, respectively, mainly due to the increase in molecular dipole moment and the reduction in thermal conductivity. Moreover, the study found a correlation between Rayleigh scattering and nonlinear absorption, providing new insights for optimizing material design.
The scientific value of this research lies in the deep understanding of the photophysical behavior and nonlinear optical mechanisms of Carmine in different environments. Its application value is reflected in the use of encapsulated samples as photosensitizers in photodynamic therapy (PDT) or in the development of optical sensors, modulators, and switches.
d) Research Highlights
- Key Findings: Encapsulation technology significantly enhanced the nonlinear optical response of Carmine, opening new avenues for its application in photonics.
- Problem Solving: By adjusting solvent polarity and additive concentrations, the issue of insufficient nonlinear optical performance of Carmine in traditional solutions was addressed.
- Methodological Innovation: Combining Z-scan technology with quantum perturbation theory, the study systematically investigated the changes in ground and excited state dipole moments of Carmine.
- Novelty: For the first time, Carmine was encapsulated in a microemulsion, demonstrating its excellent optical performance in low-polarity environments.
e) Other Valuable Information
The study also explored the color changes of Carmine under different pH conditions, providing references for the further development of pH-sensitive optical materials. Additionally, the stability and uniformity of the encapsulated samples laid the foundation for industrial production.
Summary
This paper systematically studied the photophysical and nonlinear optical properties of encapsulated Carmine, revealing the immense potential of encapsulation technology in enhancing material performance. The research findings not only enriched the fundamental theory of nonlinear optical materials but also provided important guidance for developing new photosensitizers and optical devices.