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Abstract
This study delves into the enhancement of fluorescein dye's optical and photophysical properties through the integration of cadmium sulfide (CdS) quantum dots (QDs). The research highlights the superior fluorescence intensity, resilience against photobleaching, and multiplex emission capabilities of QDs, making them ideal for advanced photonic architectures and laser technologies. The investigation focuses on the influence of CdS QDs on the photonic and laser performance attributes of fluorescein dye, exploring how QDs modulate stimulated fluorescence and enhance emission efficiency. The study systematically evaluates the impact of QD concentration on fluorescence enhancement, revealing that optimal QD loading significantly improves emission output while excessive QD concentration leads to fluorescence quenching due to aggregation. The research also examines the Förster resonance energy transfer (FRET) dynamics, quantifying the Förster radius and spectral overlap between QD emission and fluorescein absorption. The findings demonstrate that the integration of CdS QDs into fluorescein dye matrices results in enhanced optical performance, with potential applications in bio-sensing, optoelectronics, and environmental monitoring. The study concludes that the [1×10⁻⁴ M fluorescein: 5% w/w CdS QD] formulation offers optimal spectral congruence and efficient energy transfer, paving the way for advanced technological and societal applications.
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Abstract
A fluorescence enhancement strategy was successfully demonstrated by complexing fluorescein dye with cadmium sulfide (CdS) semiconductor quantum dots (QDs). The influence of varying QD concentrations on the photophysical behavior of 1 × 10⁻4 M fluorescein revealed that 5% w/w CdS QDs yielded the optimal balance for energy transfer efficiency and spectral modulation. Additionally, the effect of different dye concentrations within the complex was evaluated to understand concentration-dependent emission dynamics. Comprehensive photophysical characterization of the [Fluorescein: CdS QDs] system in ethanol was conducted, with particular focus on the underlying energy transfer mechanism. FRET-based analyses were employed to extract key parameters, including the critical transfer distance (R₀), thereby elucidating the nature and extent of QD-to-dye energy coupling. Fluorescence enhancement efficiencies were systematically studied under variable input powers using a continuous-wave blue diode laser (λ = 450 nm). The QD-dye hybrid exhibited relatively high emission efficiency, along with a significantly elevated fluorescence quantum yield.
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1 Introduction
Semiconductor quantum dots (QDs) represent a class of nanocrystalline materials distinguished by their remarkable photochemical and photophysical characteristics, which diverge notably from those observed in molecular systems or bulk semiconductors. Their superior fluorescence intensity, resilience against photobleaching, and capability for multiplex emission under single-wavelength excitation have established QDs as potent fluorescent probes with extensive applicability. The spectral tunability and electronic versatility of QDs, governed primarily by particle dimension and elemental composition, render them well-suited for integration into advanced photonic architectures, photovoltaic systems, and light-emitting diode (LED) technologies, spanning emission wavelengths across the visible to infrared regions [1, 2].
Of particular interest within this domain are II–VI semiconductor nanocrystals, such as CdSe, CdS, CdTe, ZnSe, ZnS, and ZnTe, which have been thoroughly examined for their optoelectronic potential. Advances in donor–acceptor assemblies have increasingly leveraged both metallic nanoparticles (NPs) [3‐8] and QDs [9‐11] as dynamic energy donors, capitalizing on their distinctive radiative characteristics and spectral alignment. In such configurations, QDs may function as high-efficiency dye surrogates, often conceptualized as classical dipole sources within Förster resonance energy transfer (FRET)-like frameworks. These energy transfer phenomena, governed by the principles articulated in the Fermi Golden Rule, are modulated by interspecies parameters such as donor–acceptor spatial separation, dipolar coupling orientation, and spectral concordance between donor emission and acceptor absorption profiles [12‐14].
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Fundamental to these processes are the alignments of transition dipole moments and the optical fingerprints of the interacting species. This investigation centers on the influence of CdS QDs on the photonic and laser performance attributes of fluorescein dye as a well-characterized lasing medium. Specifically, we explore the capacity of QDs to modulate stimulated fluorescence and enhance emission efficiency. The interaction between QDs and the dye matrix may facilitate fluorescence augmentation or attenuation, contingent upon factors such as donor–acceptor proximity, the kinetics of energy transference, and potential dye–QD aggregation phenomena.
2 Experimental
2.1 Materials
Cadmium acetate dihydrate and sulfur powder, oleic acid (OLA), methanol (Anlar), ethanol (Anlar), and fluorescein were purchased from (Aldrich) and used without further purification.
Cadmium sulfide (CdS) quantum dots were synthesized via a standard hot-injection approach, following the protocol outlined in reference [15]. Specifically, 20.0 mg of cadmium acetate dihydrate [Cd(Ac)2·2H2O; 0.075 mmol], 12.0 mg of elemental sulfur (0.375 mmol), and 4 mL of oleic acid (OLA) were sequentially introduced into a 25 mL three-neck round-bottom flask. The reaction mixture was subjected to continuous nitrogen purging to maintain an oxygen-free environment and heated to 250 °C, where it was held at isothermal conditions for 4 h to promote nanocrystal growth.
Upon completion of the thermal treatment and subsequent cooling to room temperature, the resulting yellow CdS nanocrystals were precipitated by the addition of excess methanol. The solids were isolated by centrifugation and subjected to five successive purification cycles to remove unreacted precursors and byproducts. The prepared quantum dots were redispersed in toluene and allowed to equilibrate for 24 h to ensure stabilization of surface ligands through conjugation dynamics.
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All solvents utilized throughout the synthesis and purification stages were anhydrous to prevent hydrolytic degradation or interference with surface chemistry. Optical characterization of the CdS QDs was conducted at ambient temperature using a UV–Vis spectrophotometer (PerkinElmer, Lambda 35), enabling assessment of their absorbance features and confirming successful synthesis.
2.3 Sample preparation
Fluorescein laser dye solutions were prepared at a fixed concentration of 1 × 10⁻4 M in ethanol, both in the absence and presence of cadmium sulfide (CdS) quantum dots (QDs), with QD loadings varying from 3 to 50% by weight. These samples were housed in quartz cuvettes possessing a 1 cm optical path length. Concurrently, a series of fluorescein dilutions spanning concentrations from 1 × 10⁻4 to 2 × 10⁻5 M was formulated to examine the influence of dye concentration on the photophysical behavior of dye–QD assemblies. These dilution series were also studied with and without embedded QDs to elucidate their role in modulating optical response.
To preserve solvent integrity and prevent volatilization during spectroscopic analysis, all cuvettes were hermetically sealed. Ethanol served as the reference medium for all formulations. The specific concentrations of fluorescein dye investigated included 1 × 10⁻4 M, 8 × 10⁻5 M, 6 × 10⁻5 M, and 2 × 10⁻5 M. This systematic matrix enabled a comprehensive evaluation of how both QD loading and dye concentration affect emission characteristics, energy transfer mechanisms, and fluorescence amplification behaviors in the resulting hybrid nanocomposite systems.
2.4 Spectra measurements
Fluorescein dye solutions were prepared at a standardized concentration of 1 × 10⁻4 M in ethanol, both with and without embedded cadmium sulfide (CdS) quantum dots (QDs), wherein QD volume fractions ranged from 3 to 50%. These preparations were enclosed in quartz cuvettes with a 1 cm optical path length and hermetically sealed to mitigate solvent evaporation during measurement.
Absorption profiles were obtained using a Camspec M501 UV–Vis spectrophotometer of spectral band width (SBW) resolution 1.0 nm and spectral range 190–1100 nm, while excitation spectra were recorded via a PF-6300 spectrofluorometer of spectral band width (SBW) resolution 0.5 nm and spectral range 200–900. The intrinsic absorbance characteristics of the synthesized CdS QDs were independently evaluated using the same spectrophotometric instrumentation to ensure consistency across measurements. The lifetimes of the fluorescein and its complex with QDs were measured by a spectrofluorometer (Model: Edinburgh Instruments FS5) using a picosecond pulsed diode laser as the light source of 450 nm wavelength. Optimal QD content within the dye matrix was determined by comparative analysis of absorption and fluorescence emission spectra. This optimal formulation was subsequently investigated across a series of fluorescein concentrations to quantify the corresponding fluorescence enhancement factors under varying dye loadings. Emission behavior of the 1 × 10⁻4 M fluorescein solutions, with and without the optimized CdS QD admixture, was further examined as a function of excitation power using a dedicated photonic setup. Samples were transversely excited with a continuous-wave diode laser emitting at 450 nm. Beam shaping was achieved via a concave lens (focal length = 10 cm) in conjunction with a cylindrical lens to generate a uniform 1 cm line focus across the sample interface. Incident laser power was continuously monitored using a Gentec QE50 power meter coupled with a 4% beam splitter. Amplified spontaneous emission (ASE) signals were collected through a convex lens (focal length = 15 cm), collimated, and subsequently directed to an Oplenic spectrophotometer interfaced with a real-time data acquisition system for spectral analysis.
3 Results and discussion
3.1 Visual characterization of prepared QDs
Transmission Electron Microscopy (TEM) image of Cadmium Sulfide (CdS) Quantum Dots (QDs) presented in Fig. 1. Whereas by visually comparing the size of the dark spots (QDs) to the 20 nm scale bar, the individual nanoparticles appear to be in the range of 2 nm to 5 nm in diameter in predominantly spherical in shape. The particles show a relatively narrow size distribution. For bulk CdS, the exciton Bohr radius is approximately 3 nm. Since the particles are in the 2 nm to 5 nm range, they are in the regime where quantum confinement effects are expected to be pronounced.
3.2 Optical properties of fluorescein with and without CdS QDs
The mean diameter of CdS quantum dots (QDs) suspended in colloidal solution was quantitatively estimated via a fourth-order polynomial fitting function derived from empirical data [15‐17], expressed as given by Eq. (1):
Where D denotes the average particle size in nanometers, and λ is the wavelength (in nm) corresponding to the first excitonic absorption peak.
To determine the molar extinction coefficient ε, Equation (2) was employed at a nominal particle diameter of 3 nm [18, 19], following the empirical formulation:
where A is the measured absorbance, I₀ and I represent the incident and transmitted light intensities, respectively, C is the molar concentration of the QDs, and d is the optical path length in centimeters.
Applying Eqs. (1) through (3), the estimated molar concentration of the prepared CdS QD solution was found to be C = 3.5 × 10–9 mol·L⁻1.
Figure 2A shows the absorption and emission spectral range of CdS QDs, while Fig. 2B shows that CdS QDs (donor) emit light with a broad peak centered around 450 nm, while the dye (acceptor) absorbs light with a main peak centered around 430–450 nm, resulting in a significantly high overlap between them. Depending on the distance between QDs and dye molecules, the FRET phenomena and their efficiencies would be integral. The energy transfer pathway would be:
Fig. 2
(A) Absorption and fluorescence spectrum of CdS QDs. (B) Emission spectra of CdS QDs (donor) and Absorption of fluorescein dye (acceptor), and (C) Schematic energy diagram of QDs (donor) and dye (acceptor)
This FRET process has experimentally led to the quenching (decrease) of the CdS QD fluorescence (around 450 nm) and a sensitized emission (increase) of the dye’s fluorescence (which would be at a longer wavelength than 450 nm, typically in the green/yellow/red region. This FRET process is represented as a schematic energy diagram in Fig. 2C.
Figure 3A illustrates the absorption spectra of fluorescein dye at a concentration of 1 × 10⁻4 M, both in its native form and in the presence of cadmium sulfide (CdS) quantum dots (QDs), recorded using 1 cm path length quartz cuvettes. The observed spectral modifications reflect underlying electronic interactions between the fluorescein molecules and the CdS QDs. Notably, the absorbance intensity of the [fluorescein: CdS QD] assembly surpasses that of pristine fluorescein in ethanol, indicating enhanced photon uptake and suggesting the formation of ground-state complexes.
Fig. 3
A Absorption spectra of a pure CdS QDs solution, b pure 1 × 10−4 M fluorescein, c–h 3–50% w/w CdS QDs in 1 × 10−4 M fluorescein. B Emission spectra of a pure 1 × 10−4 M fluorescein, b–g 3–50% w/w CdS QDs in 1 × 10−4 M fluorescein
The integration of CdS QDs leads to a measurable red shift in the dye’s absorption profile, which can be ascribed to electronic perturbations stemming from coordination interactions. In particular, electron donation from the nonbonding oxygen lone pairs of fluorescein facilitates coupling with surface sites on the QDs, thereby altering the energy landscape of the dye’s ground state. This interaction further lends credence to the occurrence of Förster resonance energy transfer (FRET), wherein excitation energy is transferred non-radiatively from the QD donor to the dye acceptor. The efficacy of this process is substantiated by significant spectral overlap between QD emission and fluorescein absorption features, as depicted in Fig. 3B.
Complementary data presented in Fig. 3B show a reduction in QD fluorescence intensity upon dye incorporation, evidencing photoluminescence quenching. This behavior underscores effective energy transfer from the excited CdS QDs to fluorescein molecules in their ground state mediated through resonant dipole–dipole interactions and reinforced by the spectral congruence highlighted in Fig. 3B.
Figure 3A illustrates a substantial increase in absorbance intensity as the volume fraction of CdS quantum dots (QDs) within the fluorescein-QD assemblies is systematically raised from 3 to 50%, an effect attributable to enhanced excitation efficiency. In parallel, Fig. 3B presents the fluorescence emission spectra of native fluorescein alongside its hybrid counterparts containing 3%, 5% w/w, 10%, 20%, 30%, and 40% QDs. These data reveal marked improvements in fluorescence output for all QD-integrated samples relative to unmodified fluorescein, corroborating efficient energy transfer and radiative amplification mechanisms.
Intriguingly, the fluorescence signal undergoes attenuation at the highest QD loading (50%), where emission intensity falls below that of pure fluorescein. This decline is likely driven by concentration-dependent phenomena such as nanoparticle aggregation or thermally mediated non-radiative quenching, both of which compromise luminescence efficiency through alternate relaxation pathways.
The collective impact of QD concentration on emission intensity and peak wavelength across fluorescein–QD composites is systematically delineated in Figs. 4A and B, offering comprehensive insight into the photophysical modulation governed by nanoparticle loading.
Fig. 4
A Emission intensity and B Wavelength of complex dye as a function of [CdS QDs] concentration
Figure 5(A&B) presents the absorption and fluorescence emission profiles of fluorescein–CdS quantum dot (QD) hybrids across a concentration gradient of fluorescein dye ranging from 1 × 10⁻4 M to 2 × 10⁻5 M, with CdS QD loading held constant at 5% w/w. The data reveal that the extent of fluorescence enhancement or attenuation is strongly influenced by the spatial relationship between dye molecules and QDs. At favorable intermolecular separations, Förster-type dipole–dipole coupling facilitates efficient non-radiative energy transfer, resulting in pronounced fluorescence amplification.
Fig. 5
A Absorption spectra of different concentrations of fluorescein without and with 5% w/w CdS QDs. B Emission spectra of different concentrations of fluorescein without and with 5% w/w CdS QDs
However, deviations from this optimal range compromise the energy transfer dynamics: excessive proximity may induce dye–QD aggregation, leading to non-radiative quenching, while excessive separation beyond the Förster radius diminishes transfer efficiency, yielding reduced emission intensity. These findings underscore the critical role of nanoscale spatial organization in modulating the optical response of dye–QD complexes. This specific type of non-radiative quenching induced by aggregation is often attributed to the formation of non-fluorescent complexes or excimer/exciplex formation (though the latter is more common in high concentrations of a single species, not necessarily aggregation). When dye molecules or QDs come into very close, physical contact (i.e., aggregate), the electronic coupling between them becomes extremely strong. This proximity allows for charge transfer or strong intermolecular interactions that act as efficient energy sinks. This energy sink, because of the aggregate structure itself often has electronic states (sometimes referred to as “dark states” or “trap states”) that are lower in energy than the excited state of the single fluorophore. The excited energy is rapidly funneled into these states and then quickly released as heat through internal vibrations, rather than being emitted as a photon.
At elevated fluorescein concentrations ranging from 1×10⁻3 to 1×10⁻2 M, in the presence of 5% w/w cadmium sulfide (CdS) quantum dots (QDs), a pronounced fluorescence quenching effect was observed. This attenuation arises from reduced intermolecular spacing and the consequent formation of aggregated dye–QD complexes. Such close proximity facilitates non-radiative decay pathways, primarily through excitonic coupling and self-quenching mechanisms, which hinder emission efficiency.
To minimize aggregation-induced effects and delineate conditions favorable for optimal energy transfer, more diluted complexes were examined, spanning dye concentrations from 2×10⁻5 to 1×10⁻4 M at a fixed QD loading of 5% w/w, as depicted in Fig. 5B. Within this regime, the average separation between fluorescein molecules and QDs approximated the Förster distance, enabling efficient resonance energy transfer and resulting in enhanced fluorescence intensity. The observed amplification is attributed to improved spectral congruence between the excitation/emission fields and fluorophores positioned within effective coupling proximity to the QD surface.
In contrast, at markedly low dye concentrations, the spatial gap between fluorophores and QDs surpasses the Förster radius (R₀), diminishing energy transfer efficiency. Under these conditions, QDs may behave as static quenchers, contributing to reduced emission output via insufficient dipolar interaction.
Figure 6 quantitatively illustrates the dependence of fluorescence enhancement factors on dye concentration within the 5% w/w QD hybrid system, elucidating the interplay between molecular spacing and photophysical performance.
Fig. 6
The Fluorescence enhancement factor (%) of different concentrations of fluorescein with 5% w/w CdS QDs
Conversely, the mechanisms underlying fluorescence enhancement or quenching within donor–acceptor assemblies are fundamentally dictated by the kinetics of Förster resonance energy transfer (FRET). As a non-radiative dipole–dipole interaction, the rate of FRET is acutely sensitive to several key parameters: the spectral overlap between the donor’s emission and the acceptor’s absorption profiles, the donor’s fluorescence quantum yield, the spatial orientation of the respective transition dipole moments, and most critically, the donor–acceptor separation distance.
A pivotal parameter in quantifying FRET efficiency is the Förster radius (R₀), defined as the interspecies distance at which the probability of energy transfer equals that of spontaneous radiative decay. This radius delineates the operative spatial range for effective energy migration between chromophores and serves as a predictive metric for the likelihood of resonance coupling within nanocomposite systems.
3.3 Förster resonance energy transfer and photophysical characterization
The Förster resonance energy transfer (FRET) radius, denoted as R0, was computed based on classical dipole–dipole interaction theory, adopting the quantitative formulation [20]:
An equivalent simplified expression is given by equations (4 and 5):
where k2 is the orientation factor (taken as 2/3 for random orientations), Φₒₙₒᵣ is the donor quantum yield (measured at 0.35, using Rhodamine B in ethanol as a standard), J(λ) is the spectral overlap integral, n is the refractive index of the medium, and Nₐ is Avogadro’s number.
The overlap integral J(λ) encapsulates the convolution between donor emission and acceptor absorption profiles, computed via equation (6):
Where FD(λ) is the normalized donor emission intensity in the absence of acceptor, and εA(λ) is the molar absorptivity of the acceptor at wavelength λ.
For CdS QDs paired with fluorescein, the calculated Förster radius was 65 Å, significantly exceeding typical collisional transfer distances (4–6 Å) [21], affirming the dominance of long-range resonant dipole–dipole coupling in the energy transfer process.
3.3.1 Absorption and emission parameters
The molar absorption coefficients (extinction coefficients) ‘‘ε’’ were calculated from the absorption measurements and Beer–Lambert law as previously mentioned in equation (3). The absorption and emission cross sections σa, σe (cm2) are then, respectively, calculated according to the formula given by Eqs. (7, 8) [22]:
Where λe is the emission wavelength, n is the refractive index of the solvent, c is the velocity of light, τf is the fluorescence lifetime, ∫E(λ) dλ is the normalized fluorescence spectrum, and φf is the quantum yield of the dyes (defined as the ratio of the number of photons emitted to the photons absorbed). \({\varphi }_{f}\) was estimated by comparing the absorption and emission spectra to those of a known standard φx according to the relation given by Eq. (9) [23].
where h is the absorption peak height, S is the area enclosed by the emission curve and wavelength axis. In the determination of fluorescence quantum yields φf (determined relative to the R6G laser dye in ethanol as reference), care was taken to keep the concentration of all the samples at levels low enough to avoid re-absorption of the emitted photons [24, 25]. The calculated values of ε, σa, σe, and φf are tabulated in Table 1.
Table 1
Photophysical Parameters of Fluorescein Dye in the Absence and Presence of CdS Quantum Dots (QDs); (ε) molecular extinction coefficient; σa and σe: absorption and emission cross section; (Λ) the attenuation length, (τcall) calculated fluorescence life time, μ12(D) the transition dipole moment, (Ef) energy yield of fluorescence, (Kr)the radiative decay rate, (Kisc) the intersystem crossing rate, f oscillator strength, φf fluorescence quantum yield
Samples
ε
L.M−1.Cm−1(104)
σa
(10–17) Cm2
σe
(10–17) Cm2
Λ (cm)
τcall (ns)
τmeas. (ns)
μ12
(D)
Ef
Kr
(108)
s−1
Kisc
(108)
s−1
f
φf
Pure (1 × 10−4 M) Flu/ ethanol
.55
2.1
4.5
0.78
7.0
3.6
6.5
0.47
0.66
.33
0.38
0.45
(1 × 10−4 M) Flu + 5% w/w QDs
.97
3.73
12.7
0.097
6.2
2.8
9.8
0.80
0.94
0.06
0.97
0.60
3.3.2 Oscillator strength and dipole moment
The oscillator strength (f) is an important characteristic of the dyes, which shows the effective number of electrons whose transition from ground to excited state gives the absorption area in the electron spectrum. Values of oscillator strength are calculated using equation (10) [26]:
A value of ~ 1 represents a strong transition, while a quantum-mechanically forbidden transition might have f ~ 0.001. The oscillator strength values of fluorescein with CdS QDs are higher than those in EtOH host which have attenuation length (cm); where The attenuation length Λ(λ): (the distance at which the original light intensity I0 reduced to (I = I0 /e) given by the Eq. (11) where (e) Euler’s number (e ~ 2.7) [27, 28]:
Where ε (λ) is the molar extinction coefficient, and c the molar concentration.
Hence, the effective number of electrons transferred from the ground to the excited states of fluorescein with CdS QDs is higher than that in EtOH. The transition dipole moment (µ12) from ground to excited state was calculated by using equation (12), where f is related to the transition moment and the Einstein coefficient by the following expressions:
Where v is the optical frequency at the maximum absorption, me and e are the mass and electrical charge of an electron, all in the international system of units (SI). A Debye (D) is the traditional non-SI unit of dipole moment. The conversion between D and SI units is 1D = 2.36 x 10-30 (coulomb. meter). The values of dipole moment on excitation of the laser dye (fluorescein) without and with CdS QDs are summarized in Table 1.
3.3.3 Fluorescence energy yield and lifetime dynamics
The energy yield of fluorescence (Ef) was also calculated by the following equation (13):
$$E_{f} = \varphi_{f} \lambda_{A} /\lambda_{f}$$
(13)
where φf is the fluorescence quantum yield, λA, λf are the maximum absorption and fluorescence wavelength. The energy yield of fluorescence values of the laser dye in different hosts was evaluated and listed in Table 1. Fluorescein with CdS QDs gave the highest quantum yield, so it has the highest energy yield and radiative decay rate with the lowest intersystem crossing rate kisc. The intersystem crossing rate constant (Kisc) and the radiative decay rate constant (Kr) are related to the quantum fluorescence yield φf for (φf ~1) by the approximate relationship presented by equations (14 & 15) [29].
$$K_{isc} = \, (1 - \varphi_{f} ) \, /\tau_{f}$$
(14)
$$K_{r} = \varphi_{f} /\tau_{f}$$
(15)
The lowest intersystem crossing rate kisc (s-1) was observed for fluorescein in CdS QDs (s-1) compared with ethanol. This behavior might be related to the highest fluorescence quantum yield for fluorescein in CdS QDs compared with ethanol (Table 1). Since excited–state lifetime gives significant information about kinetics of the intermolecular interactions such as dimer and excimer formation, energy transfer, and molecular distances [30]. The fluorescence lifetime values of the fluorescein laser dye without and with CdS QDs were calculated using the Strickler–Berg equation [31‐38]. This equation is a modification of the Einstein fundamental relationship between the probability of absorption and emission, which is applicable to polyatomic molecules in solution. Through this equation, kr can be obtained from the absorption and fluorescence spectra using equation (16):
Where \(F(\tilde{v})\) is the emission spectrum plotted on a wavenumber scale (cm-1), \(\varepsilon (\tilde{v})\) (in units of M-1 cm-1) is the molar absorption coefficient at wavenumber (in units of cm-1), and n is the refractive index of the medium. The integrals are calculated over the So↔S1 absorption and the emission spectrum.
3.3.4 Lifetime calculation via strickler–berg formalism
The fluorescence lifetime can be calculated using equation (17) [39].
$$\tau_{f = } \tau_{o} \varphi_{f}$$
(17)
The calculated fluorescence lifetime (τcall) for fluorescein laser dye without and with CdS QDs is listed in Table 1. It was found that the fluorescence lifetime of fluorescein in ethanol without QDs (7 ns) is higher than that in a CdS QDs solution (6.2 ns). So, parent fluorescein dissolved in ethanol had a higher nonradiative rate than with CdS QDs.
3.4 Laser-induced fluorescence
The stimulated fluorescence response of fluorescein dye at a concentration of 1 × 10⁻4 M was evaluated under continuous-wave excitation using a blue diode laser (λ = 450 nm), both in the absence and presence of 5% w/w cadmium sulfide (CdS) quantum dots (QDs). Emission intensities were systematically recorded as a function of input pumping power to assess the impact of QD incorporation on stimulated emission dynamics. The integration of QDs yielded notable amplification effects, enhancing the emission output across the studied power range.
Figures 7, 8 graphically depict the measured fluorescence intensities and the corresponding enhancement factors, respectively, elucidating the role of QDs in modulating power-dependent emission behavior. These findings affirm the capability of CdS QDs to enhance stimulated fluorescence via improved excitation energy harvesting and radiative transfer processes within the hybrid dye–QD matrix.
Fig. 7
Fluorescence intensities of 1 × 10–4 M fluorescein without and with 5% w/w CdS QDs as a function of input-pumping power
A Fluorescence enhancement factor of fluorescein dye as a function of input-pumping power. B The emission intensity of fluorescein without and with CdS QDs as a function of input-pumping power
As depicted in Fig. 8A, the fluorescence enhancement factor exhibits a declining trend with increasing input pumping energy, pointing to the presence of an optimal excitation power threshold. Beyond this critical point, further augmentation of pump intensity results in diminished enhancement efficacy. This attenuation may primarily be attributed to photothermal effects induced by the cadmium sulfide (CdS) quantum dots (QDs), wherein elevated energy input promotes thermal agitation and triggers morphological transformations, specifically, alterations in nanoparticle shape and size [40].
These thermally driven deformations may exert a direct influence on the optical properties of the QDs, causing shifts in their absorption profile that compromise spectral alignment and light-harvesting capability. Consequently, the efficiency of energy transfer from QDs to fluorescein dye molecules is adversely affected under high-intensity excitation, leading to fluorescence quenching and reduced emission output.
Figure 8B illustrates the dependence of output energy on excitation power for fluorescein dye solutions maintained at a fixed concentration of 1 × 10⁻4 M, with and without the incorporation of 5% w/w cadmium sulfide (CdS) quantum dots (QDs). The samples were transversely excited, producing mirrorless amplified spontaneous emission (ASE), as the pump power was systematically varied between 40 and 300 mW.
In both configurations, the emitted energy scaled proportionally with input power, a behavior attributed to the increasing population of photoexcited dye molecules under higher excitation intensities, thereby enhancing photon generation. Notably, the QD-functionalized system demonstrated consistently superior energy conversion efficiency across the entire pumping range.
This augmented performance arises from several key photophysical effects conferred by QD integration. Chief among them is the reduction in fluorescein’s fluorescence lifetime due to efficient resonance energy transfer, which accelerates excitation–emission cycling. Additionally, the presence of QDs mitigates non-radiative decay channels and curbs excited-state interactions between neighboring dye molecules, factors that collectively contribute to enhanced quantum efficiency and elevated radiative output.
4 Conclusion
A systematic investigation was undertaken to assess the photophysical characteristics of fluorescein dye and its hybrid complex with cadmium sulfide (CdS) quantum dots (QDs) in ethanol. Integration of CdS QDs into the dye matrix led to a marked enhancement in optical performance, evidencing superior emission efficiency compared to the unmodified system. The [1 × 10⁻4 M fluorescein: 5% w/w CdS QD] formulation emerged as optimal, featuring a spatial configuration conducive to maximal spectral overlap between QD emission and dye absorption bands. The presence of oleic acid ligands on the QD’s surface primarily acts as a stabilizer and insulator. They control the starting point for all subsequent dye interactions by dictating the QD’s solvent compatibility and surface reactivity, and physically separating the dye from the core, which must be carefully managed to achieve optimal energy transfer.
This optimal spectral congruence enabled efficient non-radiative energy transfer from QD donors to fluorescein acceptors, resulting in elevated fluorescence quantum yield and enhanced photostability. Förster resonance energy transfer (FRET) dynamics were rigorously analyzed, with quantitative estimation of the Förster distance (R₀) providing insight into dipole–dipole coupling efficiency.
The findings regarding the nanoscale spatial organization of the QD–Fluorescein complex and its optimized optical response are not merely academic; they hold significant promise for future technological and societal advancements. Leveraging the superior photophysical properties of Quantum Dots specifically their size-tunable emission and broad absorption in conjunction with the target-recognition capability of organic dyes like Fluorescein, this complex is poised to revolutionize several key fields; Advanced Bio-Sensing and Diagnostics in bioassays, Optoelectronics such as Quantum Dot Light-Emitting Diodes (QLEDs) or advanced solar energy harvesting devices, where efficient energy transfer is paramount for increasing device efficiency and lifespan. Societal impact (healthcare and environment), e.g., Improved Healthcare Outcomes: By enabling earlier and more precise disease diagnosis, especially cancer and infectious diseases. Environmental Monitoring: The high sensitivity of the QD-based system can be adapted for environmental sensing, allowing for the detection of trace amounts of pollutants, heavy metals, or toxins in water and air, which is critical for public health and environmental protection.
Acknowledgements
The authors extend their profound gratitude and sincere appreciation to NILES, Cairo University, for its unwavering support, generous encouragement, and invaluable contributions that made this study possible.
Declarations
Conflict of interest
The authors declare that they have no conflict of interest.
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