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Abstract
We report a whispering gallery mode (WGM)-based fiber optofluidic laser (FOFL), in which rhodamine B (RhB) in an aqueous surfactant solution of sodium dodecylbenzene sulfonate (SDBS) is used as the laser gain medium. Here, the role of SDBS is to scatter the RhB dye molecules to effectively prevent its self-association in the aqueous solution. Therefore, the fluorescence quantum yield of the used RhB dye is improved due to the enhanced solubilization, which results in a low lasing threshold of ∼2.2 µJ/mm2 when the concentration of SDBS aqueous solution reaches up to 20 mM, on par with or even better than most of the optofluidic dye lasers using RhB as the gain medium in an organic solution. We then establish a model of solubilization capacity of SDBS micelles, which successfully addresses the mechanisms of dye-surfactant interactions in the proposed FOFL system. We further apply this FOFL platform to the case of concentration sensing of the used SDBS, which exhibits a 2-order-of-magnitude improvement in sensitivity compared to the fluorescence measurement due to the signal amplification inherent to the lasing process. The proposed FOFL platform in combination with surfactant solubilization gain medium in an aqueous solution promises to enable chip-scale coherent light sources for various environmental and bio-chemical sensing applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber optofluidic laser (FOFL) [1–4], which integrates optical fiber microcavity and microfluidic channel, is highly attractive in the implementation of lab-on-a-chip systems. The FOFLs not only possess the advantages of optofluidic lasers such as low sample consumption, high sensitivity, high signal-to-noise ratio, and narrow linewidth, but have unique features of optical fiber, including ease of integration, high repeatability, and low cost. Therefore, a FOFL is a powerful platform for biochemical sensing.
In the wavelength region of 580-610 nm, rhodamine B (RhB) is an important laser dye and its aqueous solution or organic solution (such as ethanol or methanol) is usually employed as an active medium for a large variety of FOFLs [5]. However, thermal effects during the stimulated radiation process may inevitably create a temperature gradient in the RhB organic solution, which significantly reduces the stability of the laser system and limit the applications of FOFLs. In contrast, RhB aqueous solution is an excellent gain medium for FOFLs, which not only overcomes the shortcomings of the organic solvent [6] but has considerable prospects in biochemical and environmental sensing applications. Unfortunately, RhB molecules tend to self-associate at a high concentration (>∼200 µM) to form hydrophobic aggregates in the water solution [7]. Molecular aggregates of RhB, in the form of dimers or higher aggregates, will lead to fluorescence quenching and thus degrade the lasing ability of dye laser systems. Numerous prior studies have shown that the dimers are the primary factor leading to fluorescence quenching and quantum yield (QY) reduction [8]. Therefore, eliminating dyes aggregation in water solution becomes highly desirable for a dye laser system. To address this problem, surfactants are frequently used because they can help scattering the dyes and effectively prevent dye molecules from self-association. Thus, solubilization of the weakly soluble dyes is improved, which results in an increase in the fluorescence QY [8]. Although there are many studies on fluorescence properties of dye-surfactant in an aqueous solution, to the best of our knowledge, it remains unclear how a dye laser system is affected by dye-surfactant interactions.
In this article, we present a whispering gallery mode (WGM) lasing emission using a FOFL, in which a RhB aqueous solution solubilized by an anionic surfactant (sodium dodecylbenzene sulfonate, SDBS) is used as the gain medium. We first demonstrate that the lasing threshold is as low as ∼2.2 µJ/mm2 when the concentration of SDBS is up to 20 mM, on par with or even better than most of the FOFLs with RhB organic solution as the gain medium [5]. Then, we establish a model of solubilization capacity of SDBS micelles, which successfully elucidates the mechanisms of dye-surfactant interactions in the proposed FOFL system. Afterward, we carry out an experimental and theoretical investigation to characterize the lasing threshold and emission intensities under different SDBS concentrations to evaluate the sensing performance of the proposed FOFL system. We further verify that the intensity of the lasing emission and the SDBS concentration shows good linearity in the SDBS concentration range 7-10 mM, with a 2-order-of-magnitude improvement in sensitivity compared to fluorescence measurement.
2. Experiment
2.1 Materials
RhB (molecular weight 479.02 g/mol, CAS Number 81-88-9) and SDBS (molecular weight 348.48 g/mol, CAS Number 25155-30-0) were purchased from Sigma-Aldrich. Optical fiber was purchased from Nanjing Chunhui Science and Technology Industrial Co., Ltd (China). The polydimethylsiloxane (PDMS, refractive index (RI) = 1.405) chip, which was the same as Ref. [9] was supplied from HICOMP Microtech (Suzhou) Co., Ltd (China).
2.2 Experimental setup
For the study of surfactant solubilization of RhB, we use our previously reported PDMS-based fiber (Diameter = 200 µm) optofluidic ring resonator (OFRR) as the platform due to its simple fabrication, low sample consumption and easy sample delivery. Figure 1(a) illustrates the schematic of the experimental setup. The detail of the experimental setup is described in Ref. [9]. The inset of Fig. 1(a) shows the cross-section of the fiber in the microfluidic channel, which is used to depict the principle of lasing generation and sensing. The fiber is longitudinally pumped by an optical parametric oscillator (OPO) laser at 532 nm (5 ns pulse width and 20 Hz repetition rate). The pump intensity is adjusted by a continuously variable neutral density filter. The laser beam is focused on the end face of the fiber via a 75 mm lens and propagated along the fiber axis through total internal reflection (TIR) at the fiber/cladding interface. The evanescent field of the pump light extends out of the fiber surface and efficiently excites the dye molecules residing in the evanescent field region of the WGMs to lase. The emission is collected by a multimode fiber and subsequently transmitted to a spectrometer mounted with an ICCD detector (PI-Max 1024RB), which has a 0.05 nm spectral resolution when a 2400 g/mm grating is used. The direction for the emission collection is perpendicular to the fiber axis. As compared with other chip based optofluidic laser systems [10–12], the merit of the FOFL system is two-fold. Firstly, the lasing emission in the FOFL is generated by the evanescent field of the pump light. As the evanescent field of the pump beam distributes uniformly around the fiber and provides precise excitation to the dye molecules in close proximity to the fiber/cladding interface, thus the lasing emission around the fiber surface is uniform. Therefore, this pumping scheme not only effectively eliminates the background noise caused by the chip materials, but also provides a uniform excitation along the microfluidic channel. Secondly, the pump light propagates along the fiber axis makes sure a long lasing produced length, which can be used to realize cascade and multiband lasing emission with a single pump light. In the experiment, to avoid the scattering loss caused by the optical fiber surface during the lasing process, the optical fiber is cleaned with 0.5% HF before use. Figure 1(b) shows the chemical structures of RhB and SDBS, respectively. The concentration of RhB dye solution is fixed at 0.5 mM. All measurements in the experiment are conducted at room temperature (25°C).
Fig. 1. (a) Schematic diagram of the experimental setup for WGM based fiber optofluidic laser. The inset on the left bottom presents the cross-section view of the OFRR, which is used to illustrate the principle of the OFRR lasing generation and sensing. (b) Chemical structures of RhB and SDBS.
3. Results and discussion
3.1 WGM lasing with RhB gain in SDBS surfactant aqueous solution
To demonstrate the establishment of the WGM lasing, we first conduct a control experiment to confirm that the WGM lasing is indeed generated by RhB gain with the addition of SDBS in the aqueous solution (pH =7). Figure 2(a) shows the fluorescence and lasing spectra at different pump energy densities (PEDs) by utilizing a low-resolution grating (grating density = 150 g/mm). In the absence of SDBS, only spontaneous emission with a broad fluorescence spectrum can be observed despite the extremely high PED (∼100 µJ/mm2). However, even at the PED more than 30 times lower (∼3 µJ/mm2), the broad emission spectrum changes to a sharp peak in the case with an addition of 20 mM SDBS into the RhB aqueous solution, which indicates the presence of lasing action, and the lasing threshold is found to be about 2.2 µJ/mm2 (as shown in Fig. 2(b)). In contrast, the fluorescence part (without SDBS) of the RhB emission intensity increases sublinearly and gradually saturates with increasing pump intensities. In addition, the inset of Fig. 2(b) displays a notable color change from dark (0 mM SDBS) to bright (20 mM SDBS) with the increase of SDBS. This is a direct evidence that RhB dye molecules are solubilized in SDBS aqueous solution.
Fig. 2. (a) Fluorescence and lasing spectra at different PEDs. The black curve (PED = 100 µJ/mm2) shows the fluorescence emission of RhB in the absence of SDBS in an aqueous solution, while the pink curve (PED = 3 µJ/mm2) represents the lasing emission of RhB in the presence of 20 mM SDBS in an aqueous solution. (b) The spectrally integrated intensity of RhB as a function of PED for lasing emission (pink sphere) with 20 mM SDBS and fluorescence (FL, square) without SDBS in an aqueous solution. The lasing threshold is 2.2 µJ/mm2. The inset displays visually the color of RhB aqueous solution changing from dark (0 mM SDBS) to bright (20 mM SDBS). Error bars are obtained with five measurements. (c) WGM lasing spectrum (PED = 3 µJ/mm2) with 20 mM SDBS in RhB aqueous solution. The calculated mode numbers according to Eq. (1) agree well with the experimental peak position.
The characteristics of the lasing action are also studied via a high-resolution spectrometer (PED = 3 µJ/mm2) with a 2400 g/mm grating. As shown in Fig. 2(c), WGM lasing was clearly observed, as indicated by the sharp peaks in the spectrum. The interval between the adjacent lasing peaks is approximately 0.38 nm, corresponding to the theoretical spacing ∼ λ2/(2πn1a), where λ (= 598.2 nm), n1 (= 1.458), and a (= 100 µm) are the lasing central wavelength, the RI of the fiber, and the ring resonator radius, respectively. The radial mode order (l) and the azimuth mode number (n) of WGM lasing can be assigned from the explicit asymptotic formula [13]
After determining the WGM lasing characteristics of RhB in the aqueous SDBS surfactant solution, in this section, we will further investigate the WGM spectral response induced by the concentration of SDBS. The effect of SDBS on the spectra of RhB is studied at pH 7. The SDBS concentration for this purpose is varied from 4 to 20 mM with a fixed dye concentration (0.5 mM) and a PED value (25 µJ/mm2), respectively. The spectra presented in Fig. 3 clearly show that the dye-surfactant interactions, which indicates the elimination or reduction of the dye aggregates in micelles environment. When the SDBS concentration is far less than its critical micelles concentration (CMC) of 6.5 mM (see details in the SI), only a weak fluorescence spectrum is collected. However, when the concentration of SDBS is close or surpass to the CMC, the lasing emission is gradually turned on and the lasing intensities and the lasing wavelength present a concentration-dependent change. As shown in Fig. 3, when the concentration of SDBS is in the range 7-10 mM, the lasing emission intensities are significantly enhanced and the central wavelength of the lasing emission is obviously blue shifted. When the concentration of SDBS is higher than 10 mM, the lasing emission intensity increases slowly and gradually levels out. Also, the recorded central wavelength of the lasing spectrum is almost unchanged.
Fig. 3. Lasing spectra of RhB in the aqueous solution with various SDBS concentrations. The pH value of the aqueous solution is kept at pH= 7, while the PED = 25 µJ/mm2, respectively.
3.2 Principles of dye solubilization
The influence of SDBS upon the lasing spectra of RhB can be explained by the chemical structures of RhB and SDBS. As shown in Fig. 4(a), a SDBS surfactant monomer molecule consists of a hydrophilic head group (polar head) that likes water and a hydrophobic tail group (non-polar tail) that dislikes water. When the surfactant concentration reaches the CMC, the SDBS anions start to aggregate into the negatively charged micelles. The shape of micelles can be predicted by the packing parameter (Pm), which is determined as Pm = r/3 l [14]. Here, r = [3(27.4 + 26.9 nc)Nagg/(4π)]1/3 is the radius of the micelle, nc is the number of carbon atoms per hydrophobic chain of the surfactant, Nagg is the average number of surfactants in each micelle, which is 61 for SDBS from the literature [14]. l = 1.5 + 1.265nc represents the hydrophobic chain length. In general, different kinds of structures can be obtained at different Pm values [15], such as spherical micelles (Pm ≤ 1/3), cylindrical micelles (1/3 < Pm ≤ 1/2), vesicles (1/2 < Pm ≤1). In this work, the calculated Pm value is 0.27, which is lower than 1/3. Therefore, spherical micelles are mainly formed.
Fig. 4. (a) Schematic illustration of a spherical micelle of RhB/SDBS. (b) The amount of solubilized RhB in each micelle varied with the concentration of SDBS.
As schematically shown in Fig. 4(a), a spherical micelle can be subdivided into three regions [16]: (i) the interface between the micelle and the surrounding bulk water, known as the outer layer, where the hydrophilic head groups of the surfactants are present; (ii) the hydrophilic groups and the first few carbon atoms of the hydrophobic tail region, known as the palisade region; and (iii) the core of the micelle. Obviously, there is a gradient of increasing polarity from the core toward the micelle surface. Usually, the negatively charged spherical micelles can adsorb RhB cations to the palisade region via electrostatic attraction [17], which results in the solubilization of RhB dye molecules.
The molar solubilization power (SP) of a surfactant is defined as the amount of dye solubilized per mol of micellar surfactant, which can be expressed as SP = Cdye/(CSDBS - CMC) [18], where Cdye and CSDBS are the concentration of RhB and SDBS in aqueous solution, respectively. Assuming Nagg is not influenced by the solubilization of dye. The average number of dye molecules solubilized in a micelle (i.e., solubilization capacity Σ) is given by Σ = Nagg × SP [16]. According to Ref. [19], the micelle concentration Cmic can be calculated by Cmic =(CSDBS - CMC)/Nagg. Therefore, the solubilization capacity Σ can be derived as
Figure 4(b) presents the calculated values of Σ when the CSDBS is higher than the CMC of SDBS. From the results shown in Fig. 4(b), when the CSDBS increases from 7 to 10 mM, the Σ decreases remarkably from ∼61 to ∼8, indicating the elimination or reduction of the dye aggregates in micelles environment. This leads to a clear increasing of lasing intensity in Fig. 3. However, when the CSDBSreaches 10 mM, the Σ reaches an optimum level. Accordingly, the kinetic equilibrium between the dye molecules and micelles are established. On further increase in SDBS concentration, all dye molecules are absorbed into micelles as monomer molecules. Therefore, the lasing emission intensity in Fig. 3 increases slowly and gradually approaches a saturated value.
The electrostatic interactions between the anionic surfactant and the cationic dye molecules can also be verified by the solvatochromism in the inset of Fig. 2(b) and the blueshifts in the lasing spectra shown in Fig. 3, which indicates that the microenvironment of the RhB molecules has been changed. With the increase of SDBS concentration, the surfactant molecules will transit from free molecules to micelles. Accordingly, the environment of RhB molecules will transfer from a water-polar solution to a non-polar micellar solution, and then the central wavelength of the lasing spectra is blue shifted.
3.3 Influence of SDBS concentration on lasing threshold
According to the aforementioned analysis, we consider that the change of lasing emission intensity under different SDBS concentrations in Fig. 3 is mainly induced by the variation of solubility of RhB in an aqueous solution, which in turn leads to the difference of the lasing threshold. Figure 5(a) plots the integrated lasing intensity under different SDBS concentrations as a function of PED, which indicates that the lasing threshold decreases with the increasing concentration of SDBS. Figure 5(b) quantitatively presents the change of the lasing threshold with various concentrations of SDBS, which shows that the lasing threshold is significantly lowered about 6-fold in the concentration range 7-10 mM. However, the lasing threshold decreases slowly and gradually tends to be a saturated value when the concentration of SDBS is above 10 mM.
Fig. 5. (a) The integrated lasing emission intensities under different SDBS concentrations as a function of PED. The solid line represents a linear fitting above the threshold. The lasing threshold is about 23.6 µJ/mm2 (7 mM), 9.3 µJ/mm2 (8 mM), 6.5 µJ/mm2 (9 mM), 4.0 µJ/mm2 (10 mM), 2.6 µJ/mm2 (15 mM), and 2.2 µJ/mm2 (20 mM), respectively. (b) Lasing threshold varied with the SDBS concentration. Error bars are obtained with five measurements.
The threshold dependence on SDBS concentration can be modeled using a four-energy-level laser system, in which the lasing threshold, Ith, can be written as [20]
where γ(λ) is the fraction of the excited molecule population at the lasing threshold, which can be written asIn a micellar environment, a change in any of the parameters in Eq. (4), i.e., absorption cross-section (σa) of dye, fluorescence lifetime (τ), and fluorescence quantum yield (Φ), will affect the lasing threshold. Since the RI of the solvent environment around the dye molecules tends to be uniform due to the dense distribution of micelles at a high SDBS concentration, then the effective σa(Qabs) of the dye molecules can be considered on par with the case without SDBS (see details in the SI). In addition, the dye-surfactant ion pairs are not bound by covalent interaction, the τ of RhB molecules (as shown in Fig. 6(b)) can be considered as independent of SDBS concentrations (see details in the SI). Therefore, the effect of SDBS surfactant on the lasing threshold is mainly by separating the dimers into monomers to lower the concentration quenching effect and thus improve the quantum yield Φ of the dye. As shown in Fig. 6(c), the quantum yield Φ shows very distinct dependence on the concentration of SDBS and increases from 0.16 to 0.63 in the SDBS concentration range 7-10 mM, which is improved ∼ 4-fold and results in a significant increase of σe(λ) from 4.96×10−17 to 1.56×10−16 cm2 at the lasing wavelength 598 nm (see details in the SI). Accordingly, the lasing threshold is drastically reduced. Beyond 10 mM, the Φ experiences a minor increase and eventually levels off, and then the lasing threshold gradually tends to be a saturated value. As shown in Fig. 6(d), the normalized experimental and theoretical threshold behaviors show a very good qualitative agreement.
Fig. 6. (a) Absorption of RhB aqueous solution under different SDBS concentrations. (b) The τ of RhB aqueous solution under different SDBS concentrations. (c) Quantum yield of RhB aqueous solution under different SDBS concentrations. (d) Comparison of the normalized experimental and theoretical threshold. Black squares are experimental results. The solid line is the calculation results of lasing threshold. Error bars are obtained by five repeated measurements.
3.4 Highly sensitive sensing for SDBS concentration using a FOFL
In this section, we will discuss the application of the proposed FOFL platform in chemical sensing by taking the light intensity variation caused by the SDBS concentration as an example. Figure 7 shows the integrated lasing spectral intensity (I) versus the SDBS concentration (CSDBS) under the same experimental conditions in Fig. 3. As shown in Fig. 7, the light intensity exhibits a very distinct dependence on SDBS concentration in the range 7-10 mM. The relationship between the lasing intensity and the concentration is linear, with a remarkable increase in intensity ∼ 50-fold. Beyond 10 mM, the lasing intensity goes through a small increase and gradually levels out. Thus, the relationship between the lasing intensity and the concentration is nonlinear in this area. It is noticeable that the photostability is independent on the SDBS concentration, the slight lasing emission variation can be mainly attributed to the fluctuations in the OPO output pulse energies.
Fig. 7. Normalized intensity for both lasing and fluorescence signal at various concentrations of SDBS. Error bars are obtained by five repeated measurements.
The sensitivity of the proposed FOFL can be defined as
According to Eq. (5), we acquire that the sensitivity (S) is as high as 114 at CSDBS = 7 mM. We also carry out a contrast experiment on the fluorescence intensity under different SDBS concentrations, in which a continuous-wave (CW) laser at 532 nm is employed as the exciting source. As presented in Fig. 7, the obtained results are very similar to the previous one in the lasing experiment, i.e., when the concentration of SDBS increases from 7 to 10 mM, the relationship between the light intensity and the concentration is also linear, but the increased intensity is just about 1.6-fold. Beyond 10 mM, the light intensity increases nonlinearly and gradually reach a saturated value. The calculated sensitivity (S) at CSDBS = 7 mM is ∼ 1.5. Obviously, the sensitivity of the lasing based detection is nearly two-orders-of-magnitude higher than that of fluorescence.
4. Conclusion
In conclusion, we have demonstrated the feasibility of SDBS solubilization of RhB gain in an aqueous solution and successfully achieved low threshold lasing emission in a WGM-based fiber optofluidic laser system. We consider that the decrease of lasing threshold is mainly attributed to the increase of quantum efficiency of RhB, as induced by the elimination or reduction of the dye aggregates in micelles environment, i.e., when the concentration of SDBS is in the range of 7-10 mM, the dye aggregates are drastically separated and solubilized into the increasing micelles. Therefore, the quantum efficiency of RhB increases rapidly, resulting in a significant reduce in the lasing threshold. After 10 mM, all dye molecules are absorbed into micelles as monomer molecules. Then, the quantum efficiency of RhB increases slowly and gradually tends to be a constant. Accordingly, the lasing threshold exhibits the corresponding saturation behavior. The laser threshold is as low as 2.2 µJ/mm2 when the concentration of SDBS is up to 20 mM, on par with or even better than most of the optofluidic dye lasers with RhB organic solution as the gain medium. The dependent studies of lasing and fluorescence on SDBS concentration suggest that the sensitivity of lasing emission based on the proposed FOFL system is about 2-order-of-magnitude improvement compared to fluorescence measurement in the SDBS concentration range of 7-10 mM. Our method can be further extended to a wide range of weakly water-soluble dyes, and thus it can provide a powerful strategy for various biochemical and environmental applications.
Funding
Joint Key Project of Yunnan Province of China (2018FY001-020); Young and Middle-aged Academic Leaders in Yunnan Province (Reserve Talents), China (2018HB029); National Natural Science Foundation of China (11864045).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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