Theoretical Investigation on a Novel Fluorescent Probe for Sulfite: Structural, Electronic, and Spectral Characteristics Underlying Detection

Abstract

This study focused on theoretical investigation of a fluorescent probe formed by conjugating a benzo[e]indolium fluorophore and a 4-(4-methylpiperazin-1-yl) benzene moiety via an ethylene linkage for sulfite detection. The Michael addition of sulfite to the ethylene bond altered the geometric and electronic structures of the probe, disrupting the charge transfer pathway, which was reflected in changes to the UV absorption and fluorescence emission spectra. The probe showed a fast response, low limits of detection, and high precision for sulfite detection, providing a promising technology for food safety and biological analysis. Theoretical calculations in this research, including density of states analysis, UV-Vis absorption, electron density distribution, and reorganization energy calculations, were in good agreement with experimental findings and provide the insights for designing the new functional fluorescent probe.

Keywords:
fluorescent probe; sulfite; quantum mechanics; electron excitation; charge transfer

Introduction

Sulfite detection is crucial for food safety and biological system. Sulfites are widely used as preservatives and antioxidants in the food industry.¹-⁴ However, the excessive use of sulfites can be harmful to human health. For example, it can lead to allergic reactions, respiratory problems, and even damage to the digestive system.⁵-⁷ By detecting sulfite levels in food, it is possible to ensure that the amount of sulfites used complies with safety standards, thus protecting the health of the consumers. Appropriate levels of sulfite can help prevent the oxidation of food ingredients, delay the browning reaction, and inhibit the growth of microorganisms. This helps to maintain the color, flavor, and nutritional value of food. Detecting sulfite levels can ensure that the food is preserved under optimal conditions and its quality is maintained throughout the storage and distribution process. Sulfite is involved in various biological processes, such as sulfur metabolism. Detecting sulfite in biological samples can help researchers understand the role of sulfite in these processes and how it affects the overall physiological function of organisms.⁸-¹⁰ For example, sulfite can affect photosynthesis and antioxidant defense systems in plants.¹¹ By detecting sulfite levels, we can better understand the mechanisms by which plants respond to environmental stresses. Besides, the abnormal sulfite levels in biological fluids such as blood and urine may be associated with certain diseases. For instance, in patients with some liver and kidney diseases, the metabolism and excretion of sulfite may be affected, leading to changes in sulfite levels. Detecting sulfite levels in these biological samples can provide important diagnostic and monitoring information for diseases, helping doctors to understand the condition of the patient and evaluate the effectiveness of treatment.¹²-¹⁴

Fluorescent probes can detect sulfite at very low concentrations, often in the nanomolar range.¹³,¹⁵-¹⁹ This high sensitivity allows for the early detection of sulfite in various samples, even when present in trace amounts. Some probes can detect sulfite levels as low as 10 20 nM, which is crucial for applications where precise quantification of low-level sulfite is required, such as in biological systems or in the analysis of food products with strict sulfite limits.²⁰ Fluorescent probes can be designed to specifically target sulfite ions, minimizing interference from other substances commonly found in the sample matrix. This selectivity is achieved through the careful design of the probe’s molecular structure, which often incorporates specific recognition sites or functional groups that interact preferentially with sulfite.²¹,²² As a result, the probe can accurately detect sulfite even in the presence of other ions and molecules, providing reliable and accurate results. Fluorescent probes typically exhibit a fast response to sulfite (often within a few minutes or even seconds) allowing for immediate detection and analysis. This rapid response is beneficial for real-time monitoring applications, such as in-line process monitoring in the food industry or in vivo detection in biological systems, where timely information about sulfite levels is essential. Fluorescent probes can often be used under mild conditions, which are less likely to damage the sample or interfere with the biological or chemical processes being studied. In biological applications, this allows for the detection of sulfite in living cells, tissues, or organisms without causing significant harm or altering their normal functions. Additionally, the non-invasive nature of fluorescent probes enables repeated measurements over time, providing valuable kinetic information about sulfite dynamics.²⁰,²³-²⁸

A novel fluorescent probe was built by conjugating a benzo[e]indolium fluorophore and a 4-(4-methylpiperazin-1-yl) benzene moiety via an ethylene linkage.²⁹ This probe enabled rapid and accurate detection of sulfite, which was critical for food safety and human health. This study revealed that the Michael addition reaction between sulfite and the ethylene bond altered the geometric and electronic structures of the probe molecule. This disruption led to the breakdown of the charge transfer pathway during electronic excitation, subsequently causing changes in the UV absorption and fluorescence emission spectra of the product. Experimental results demonstrated that these spectral changes exhibited a strong linear correlation with sulfite concentration. The probe featured a fast response and low limits of detection, making it a versatile fluorescent detection tool with high precision, sensitivity, and multi-dimensional capabilities for sulfite detection. This provided a promising technology for food safety and biological analysis. Understanding the mechanism through calculations can guide the design of experiments to further study the probe-target interaction and build the new functional fluorescent sensors.³⁰,³¹

Methodology

Molecular structure analysis

The most stable ground-state structure S₀ of the probe molecule was optimized under the combination of PBE0/def2-TZVPD with D3 dispersion through the Gaussian 16 program.³²,³³ Other functionals such as CAM B3LYP were also employed for relevant calculations, yielding similar results. Therefore, for the sake of conciseness, discussions in this paper were all based on the calculation results of the PBE0 functional. The rationality of these two functionals for applications in similar organic molecules could be referred to previous studies.³⁴,³⁵ Density of states (DOS) analysis through Multiwfn 3.8(dev) code was carried out for the S₀ structure of the probe and its sulfite-bound product to understand the orbital localization and electronic transitions.³⁶,³⁷

Spectral analysis

UV-Vis absorption spectra of the probe and its sulfite adduct were simulated. The absorption peak at the maximum wavelength was analyzed to confirm charge transfer character. Electron density distribution analysis was performed during S₀-S₁ excitation for both the probe and its sulfite adduct. Reorganization energy between ground state S₀ and first excited state S₁ of the probe and sulfite adduct was calculated. Some of the figures were delineated through VMD 1.9.3 software.³⁸

Results and Discussion

The most stable ground-state structure S₀ of the optimized probe molecule is shown in the Figure 1.

Figure 1
S₀ structure of the optimized probe molecule.

The dual descriptor was a very popular method defined within the framework of conceptual density functional theory for predicting electrophilic and nucleophilic reaction sites.³⁹ Recently, the dual descriptor potential was proposed by Martínez-Araya.⁴⁰ The paper pointed out that it had certain advantages over the dual descriptor in predicting reaction sites and was also more rigorous in principle. The calculated dual descriptor potential of the probe molecule indicated the No. 37 C atom of the C=C bond (which has the biggest negative dual descriptor potential) was the most probable electrophilic reaction site as shown in Figure 2, which suggested it would react with the sulfite.

Figure 2
The calculated dual descriptor potential of the probe molecule (green and orange isosurface stand for positive and negative dual descriptor potential, respectively).

DOS analysis of the probe’s S₀ structure as shown in Figure 3a revealed that the first and second highest occupied molecular orbitals (HOMO, HOMO-1) were primarily localized on a 4-(4-methylpiperazin-1-yl) benzene moiety, while the lowest unoccupied molecular orbital (LUMO) was centered on a benzo[e]indolium fluorophore moiety. The S₀-S₁ electronic transition occurred via HOMO, HOMO-1→LUMO, displaying typical charge transfer characteristics. In contrast, the DOS of the sulfite-bound product as shown in Figure 3b showed that HOMO, HOMO-1, and LUMO orbitals were all localized on 4-(4-methylpiperazin-1-yl) benzene moiety. As a result, S₀ S₁ transitions were confined to 4-(4-methylpiperazin-1 yl) benzene moiety with local excitation properties.

Figure 3
DOS of (a) probe and (b) sulfite adduct.

Investigating the electronic excitation processes of the probe and its sulfite adduct was crucial for understanding the fluorescence detection mechanism. The UV-Vis absorption of the probe primarily occurred in its main molecular plane, as evident from simulated spectra as shown in Figure 4a. In the sulfite adduct structure, the introduction of HSO₃^- disrupted the planar geometry of the molecule. UV-Vis spectra of the adduct shown in Figure 4b acquired from all orientations revealed contributions from non-planar absorption components.

Figure 4
Simulated UV-Vis absorption spectrum of (a) probe and (b) sulfite adduct based on PBE0/def2TZVPD method.

Analysis of the absorption peak at the maximum wavelength (480 nm) confirmed significant charge transfer character in the S₀-S₁ excitation process. Electron density distribution analysis as shown in Figure 5a revealed charge transfer between benzo[e]indolium fluorophore and 4-(4-methylpiperazin-1-yl) benzene moieties. Structural comparisons between the S₀ and S₁ states of the probe showed that conformational changes were localized to 4-(4-methylpiperazin-1-yl) benzene, which became significantly displaced from the benzo[e]indolium fluorophore plane upon excitation. This was clearly visualized in structural overlay diagrams (Figure 6a).

Figure 5
Electron density distribution during the S₀-S₁ excitation of (a) probe and (b) sulfite adduct (the orange and green isosurfaces in the figure represent the hole and electron distribution, respectively).

Figure 6
Structural comparisons between the S₀ and S₁ states of (a) probe and (b) sulfite adduct.

Electron density distribution analysis during the S₀-S₁ excitation of the sulfite adduct revealed that the introduction of HSO₃^- blocked the charge transfer pathway present in the probe’s S₀-S₁ transition, resulting in localized excitation confined to 4-(4-methylpiperazin-1-yl) benzene moiety as shown in Figure 5b. This structural modification caused significant shifts in both UV absorption and fluorescence emission wavelengths. The emission fluorescence maximum wavelength shifted from 595 to 473 nm. Monitoring the fluorescence intensity ratio between the probe and sulfite adduct thus provided an effective method to quantify sulfite concentration in solution. Structural comparisons between the S₀ and S₁ states of the sulfite adduct as shown in Figure 6b showed that conformational changes were distributed throughout the molecule, unlike the probe where changes were confined to moiety 4-(4-methylpiperazin-1-yl) benzene. The maximum absorption wavelength in the UV Vis spectrum of the sulfite adduct was reduced by about 150 nm compared to the probe, enabling highly sensitive sulfite detection via spectral shifts.

Reorganization energy is a key quantity for calculating electron transfer rates based on Marcus theory, specifically divided into inner reorganization energy and outer reorganization energy. The former measures the energy change of the system caused by geometric structure relaxation after electron gain/loss (or more generally, after changes in electronic states).⁴¹ The inner reorganization energy between ground state S₀ and first excited state S₁ of the probe and sulfite adduct were calculated, respectively. It can be seen in Figure 7 that the main vibration contribution to the reorganization energy was the torsion of the 4-(4-methylpiperazin-1-yl) benzene moiety (126 cm^-1) in the probe and the whole molecule (223 cm^-1) in the sulfite adduct. This result was consistent with conformational changes between the S₀ and S₁ states of the probe and sulfite adduct.

Figure 7
The reorganization energy between S₀ and S₁ of the (a) probe and (b) sulfite adduct.

To further investigate the influence of the changes in the electronic structure of the probe molecule before and after reacting with sulfite on its electron excitation, the distribution of π electrons in the probe and its product was analyzed. As can be seen from the Figure 8, the π electrons on the C=C bond in the probe molecule connect the π electrons throughout the entire molecule, enabling the electron excitation process to involve the whole molecule. However, in the sulfite adduct, due to the introduction of sulfite, the C=C bond was reduced, causing the interruption of the π electron distribution. As a result, the electron excitation was only concentrated in 4-(4-methylpiperazin-1-yl) benzene moiety, leading to a significant change in its fluorescence emission wavelength. This was also the reason why the probe can detect the concentration of sulfite in the environment through the change in the fluorescence wavelength.

Figure 8
π electrons distribution in the (a) probe and (b) sulfite adduct.

The wavelengths and orbital transition details for the lowest-energy absorption and fluorescence emission (S₀ S₁) of the probe and its sulfite adduct were reported in the Tables 1 and 2, respectively. The results demonstrated excellent agreement between theoretical calculations and experimental findings.

Table 1
Main electron excitation processes in the probe and sulfite adduct molecule

Probe/sensing product	Electronic transition	Excitation energy theoretical/ experimental / nm	Oscillator strength	Compositiona	CIb
Probe	S0 → S1	475/480	0.8013	H → L H-1 → L	0.2721 0.7653
Sulfite adduct	S0 → S1	323/300	0.7962	H → L H-1 → L	0.6804 0.3482

• ^a
  H stand for HOMO and L stands for LUMO.
• ^b
  Coefficient of the wave function for each excitation was in absolute value.

Table 2
Main electron emission processes in the probe and sulfite adduct molecule

Probe/sensing product	Electronic transition	Emission energy theoretical/ experimental / nm	Oscillator strength	Compositiona	CIb
Probe	S1 → S0	587/595	0.7926	L → H L → H-1	0.1896 0.7012
Sulfite adduct	S1 → S0	461/473	0.7524	L → H L → H-1	0.7024 0.3057

• ^a
  H stand for HOMO and L stands for LUMO.
• ^b
  Coefficient of the wave function for each excitation was in absolute value.

Conclusions

This study theoretically presents a novel fluorescent probe for sulfite detection based on the conjugation of a benzo[e]indolium fluorophore and a 4-(4-methylpiperazin-1-yl) benzene moiety via an ethylene linkage. Theoretical calculations, comparing with the experimental results, reveal that the Michael addition of sulfite to the ethylene bond disrupts the probe’s charge transfer pathway, leading to significant shifts in UV absorption and fluorescence emission spectra. The theoretical calculations accurately explain the fluorescence detection mechanism. Dual descriptor potential analysis accurately identified the electrophilic C=C bond (No. 37 C atom) as the reactive site for sulfite addition. Density of states and electron density distribution analyses confirmed that sulfite binding localized electronic transitions to the 4-(4-methylpiperazin-1-yl) benzene moiety, altering fluorescence properties. Calculations showed increased molecular flexibility in the sulfite adduct (223 cm^-1 vs. 126 cm^-1 in the probe), consistent with observed conformational changes. This technology holds great promise for applications in food safety and biological analysis. It can provide multi dimensional information through changes in UV absorption and fluorescence emission spectra, as well as electron density distribution and conformational changes. All the theoretical results help to comprehensively understand the interaction mechanism between the probe and sulfite, and provide a more complete technical means for sulfite detection.

Data Availability Statement

Any additional data may be obtained from the corresponding authors upon reasonable request.

Acknowledgments

This work was funded by Natural Science Foundation of Liaoning Province (2024-MSLH-147, LJ212410160071).

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Edited by

• Editor handled this article:
  Paula Homem-de-Mello (Executive)

Publication Dates

• Publication in this collection
  08 Sept 2025
• Date of issue
  2025

History

• Received
  28 May 2025
• Accepted
  31 July 2025