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Abstract
Nitrite ion ($\textrm{NO}_{2}^{-}$) is a common contaminant that can significantly threaten human health and the environment. In this study, we demonstrate a chemical sensing platform to monitor the nitrite concentration using a fiber optofluidic laser (FOFL). An optical fiber, integrated into a microchannel, is used both as an optical micro-cavity and the sensing element. Rhodamine 6 G (Rh6G) in an aqueous micellar solution is used as the laser gain medium. The light intensity change of the lasing spectra is employed as an indicator for the $\textrm{NO}_{2}^{-}$ ion concentration sensing. The lasing properties under different $\textrm{NO}_{2}^{-}$ ion concentrations are experimentally and theoretically investigated to examine the sensing performance of the FOFL. The results show that the limit detection of the FOFL sensor is 0.54 µM, which is 2-order-of-magnitude lower than fluorescence measurement. The sensing mechanism of Rh6G for $\textrm{NO}_{2}^{-}$ detection is studied by using density functional theory (DFT). The calculation results indicate that nitrite influences the electronic distribution of Rh6G based on the heavy atom effect, which leads to the fluorescence quenching of Rh6G in the excited state. In addition, the detection system exhibits favorable selectivity for $\textrm{NO}_{2}^{-}$ ions.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nitrite ion ($\textrm{NO}_{2}^{-}$) plays a unique and vital role in daily life and the environment within a specific concentration range [1–3]. However, nitrite shows interactions with the amines in the stomach to form the severe carcinogenic compound N-nitrosamines. Moreover, the normal hemoglobin in human blood is oxidized to methemoglobin by $\textrm{NO}_{2}^{-}$, resulting in the loss of oxygen-carrying capacity for hemoglobin. Thus, an excessive dose of nitrite could cause human poisoning [4]. Hence, monitoring the level of nitrite is of great significance to protect human health and prevent environmental pollution. Various types of sensors have been applied for highly sensitive and selective nitrite detection, such as fluorescence sensing [5], electrochemical platform [6], colorimetric assay technology [7], and spectrophotometry method [8]. Fluorescence sensors have been widely used in biochemical detection owing to their simplicity, selectivity, and fast response [9]. However, several drawbacks still exist in the fluorescence sensors, such as the large consumption of reagents, and detection in the open system, which makes it very challenging to monitor hazardous, toxic, and volatile solutions, resulting in limiting application in biochemical sensing.
Fiber optofluidic laser (FOFL) integrates optical fiber microcavities and liquid gain medium in the microfluidic channel and is typically used as sensors or on-chip light sources [10–12]. The FOFLs not only possess the natural advantages of optical fiber, such as ease of integration, high repeatability, and low cost but also have plentiful distinctive performance advantages, including miniaturization, high sensitivity, high signal-to-noise ratio, narrow linewidth, low sample consumption, low limit of detection (LOD), effective liquid manipulation, and replacement [13], which overcomes the shortcomings of the fluorescence sensors. Hence, FOFL provides an excellent tool for chemical analysis, biological sensing, and environmental monitoring. The optofluidic laser systems have been widely used as a platform for the detection of chemical and biological, such as Cu2+ [14], S2− [15], and pH [16], which has been achieved by using different sensing materials, such as pyrene, anthracene, and rhodamine [17–19]. Among numerous fluorescent organic dyes, thanks to the excellent properties of photo-stability and photo-physical, rhodamine can be used not only as a laser gain medium but also as a sensing probe. Therefore, it is a satisfactory fluorophore in optofluidic laser sensors. However, the deficiency of rhodamine dyes is low solubility in aqueous solutions hindering the application of biochemical detection [20,21].
Here, we demonstrate a whispering gallery mode (WGM) lasing sensor based on a FOFL for nitrite detection, in which Rhodamine 6 G (Rh6G) solubilized in an aqueous micelle environment formed by an anionic surfactant (sodium dodecyl sulfate, SDS), is served as the gain medium. By the unique characteristics of WGMs and microfluidic, some advantages possessed by WGM lasing-based FOFL sensors include: (i) the lasing emission of WGM-based FOFL is generated by the evanescent field excitation of the pump light. As the evanescent field of the pump beam distributes uniformly around the fiber surface and provides precise excitation to the active medium close to the fiber/liquid-cladding interface, thus the lasing emission around the fiber is uniform. Therefore, this pumping method not only effectively eliminates the background noise caused by the chip materials, but also provides a uniform excitation along the fiber axis; (ii) The analyte is enclosed in a microfluidic channel of the FOFL system, which provides an ideal detection platform for the hazardous, toxic, and volatile solutions; (iii) the FOFL sensor only consumes a small volume of sample and leads to a low cost per detect due to the small size of the microfluid channel; (iv) The operation of active WGM is more convenient compared with passive WGM microcavity. We first achieve lasing emission with the lasing threshold as low as ∼1.2 µJ/mm2 in the micelle-dye system. Then, the dependence of the laser intensity on the $\textrm{NO}_{2}^{-}$ concentration is monitored to achieve quantitative nitrite detection. The sensing performance is evaluated by recording the lasing properties of Rh6G in nitrite. A variety of interfering ions are used to study the selectivity of the proposed FOFL sensor. The testing results show that the fiber optofluidic laser chemosensor has a satisfactory selectivity for nitrite. More significantly, the LOD is improved with a 2-order-of-magnitude compared to fluorescence measurement. Furthermore, Density functional theory (DFT) analysis is performed to verify the mechanism of the fluorescence quenching effect for the detection of nitrite.
2. Experiment
2.1. Reagents and materials
All materials used are purchased without further purification or modification unless specially mentioned. Rh6G (purity: 99%, CAS Number 989-38-8) and SDS (purity: 99%, CAS Number 151-21-3) are obtained from Sigma-Aldrich. Potassium Iodide (KI, purity: 99%), Potassium nitrite (KNO2, purity: 97%), and Hydrochloric acid (HCl) are obtained from Macklin (China). Optical fiber (Transmittance per meter > 99%) is purchased from Nanjing Chunhui Science and Technology Industrial Co., Ltd (China). The polydimethylsiloxane (PDMS, refractive index (RI) = 1.405) chip, which is the same as Ref. [22], is supplied from HICOMP Microtech (Suzhou) Co., Ltd (China). Deionized (DI) water is obtained using an ultrapure system (EU-LS-100TJ, China). In the experiment, we also prepare some salts of NaCl, KBr, Na2SO4, and CH3COONa to study the selectivity of the proposed FOFL sensor.
2.2. Experimental setup
The schematic diagram of the lasing experimental platform used for nitrite detection is illustrated in Fig. 1(a). The microfluidic chip is fabricated via soft lithography procedures. An optical fiber with a diameter of 125 µm is integrated into the microfluidic channel made with PDMS. The size of the chip and the microfluidic channel is 20 mm × 10 mm × 2 mm (l × w × h) and 5 mm × 300 µm × 300 µm, respectively. The Rh6G gain medium in an aqueous micellar solution is injected into the microfluidic channel with a syringe (PHD 2000, Harvard Apparatus) and flowed through the microfluidic channel (flow rate: 1 µL/min). Then, a pulsed laser (wavelength of 532 nm) generated by an optical parametric oscillator (OPO) laser is employed as the pump source. The pump beam is longitudinally coupled into the fiber along the fiber axis by an optical lens at a focal length of 75 mm. When the dye molecule is excited, the stimulated emission is reflected back and forth in the ring resonator cavity formed by a circular cross-section of the fiber and amplified [23]. The pump laser and the residual fluorescence are filtered out by a long-pass filter (cutoff wavelength of 550 nm). A spectrometer (Spectrapro 500i) mounted with an ICCD detector (PI-Max 1024RB) is used for collecting the filtered laser emission spectra. The direction for the emission collection is perpendicular to the fiber axis. Details of the experimental setup are provided in Fig. S1. Each group of experiments is repeated at least five times to avoid artifacts. All measurements in the experiment are conducted at room temperature (25°C).
Fig. 1. (a) Schematic diagram of the experimental platform. (b) Schematic diagram of interaction quenching of nitrite on Rh6G.
3. Results and discussion
3.1. WGM spectral response of Rh6G in an aqueous micellar solution
To prove the lasing behavior of the WGM lasing using Rh6G dye in an aqueous micellar solution as laser gain, the characteristics of the WGM lasing action are studied. The concentration of SDS is fixed at 20 mM, and the concentration of Rh6G is 0.5 mM as it has relatively high solubilization in an aqueous micellar solution formed by SDS (see details in Supplement 1). Firstly, the emission spectra under different pump energy densities (PEDs) with a 2400 g/mm grating are shown in Fig. 2(a), which presents the turn-on process of the lasing emission. The free spectral range (FSR) is an important parameter of WGM lasing. The FSR experimentally measured to be 0.55 nm by the spectra, which is in good agreement with the theoretical prediction by using FSR = λ2/(2πn1a) of WGM lasing, where λ is the lasing central wavelength (= 564 nm), n1 is the fiber refractive index (= 1.458), and a is the radius of the fiber (= 62.5 µm) [24]. Then, the laser threshold of the FOFL is characterized. The dependence of the spectrally integrated lasing intensities versus PEDs is given in Fig. 2(b), revealing a clear threshold behavior at 1.2 µJ/mm2, where lasing will only occur beyond the energy density. All these demonstrate a mechanism characteristic of WGM lasing emission with Rh6G in an aqueous micellar solution as the gain medium.
Fig. 2. (a) Lasing spectra of Rh6G in an aqueous micellar solution with different pump energy densities (PEDs). (b) Lasing spectrally integrated intensity under different PEDs. The error bars represent standard deviation of five repeated measurements.
3.2. Detection of $\textrm{NO}_{2}^{-}$ ions
According to the aforementioned analysis, Rh6G-based FOFL in an aqueous micellar solution may achieve low threshold lasing emission, which provides an excellent candidate for lasing sensing. To assess the feasibility of Rh6G as the sensing probes, the detection of nitrite in an aqueous micellar solution is performed at the optimal lasing concentration of 0.5 mM acidized Rh6G. The mixture of nitrite solutions at various concentrations ranging from 0 to 500 µM is prepared, which is mixed with acidized Rh6G and KI at a different volume ratio. The mixture solution is injected into the microfluidic channel using a syringe pump (flow rate = 1.0 µL/min) and the lasing spectra are recorded under a constant PED (25 µJ/mm2).
The lasing spectra with different nitrite concentration using is presented in Fig. 3(a). The lasing intensity gradually decreases as the concentration of nitrite increases. From those spectra, the lasing wavelength remains almost constant during the concentration of nitrite is changed. The spectrally integrate intensity as a function of the nitrite concentration is shown in Fig. 3(b). The lasing intensity is linearly decreased as a function of the concentration of nitrite when the nitrite is below 200 µM. However, when the concentration of nitrite is beyond 200 µM, the lasing emission turns off and the spectral intensity gradually approaches a saturated value in this region. We further systematically study the lasing threshold of Rh6G varied with nitrite concentration in an aqueous micellar solution. As the nitrite concentration increased, the lasing threshold decreased gradually, as shown in Fig. 3(c). However, In the presence of a concentration of nitrite higher than 200 µM, only spontaneous emission with a broad fluorescence spectrum can be observed despite the extremely high PED (∼100 µJ/mm2).
Fig. 3. (a) Lasing spectra of Rh6G in an aqueous micellar solution with different concentration of nitrite. (b) The normalized lasing intensity of Rh6G in an aqueous micellar solution varied with nitrite concentration. (c) The lasing threshold for Rh6G in an aqueous micellar solution and their dependence on the nitrite concentration. Error bars are standard deviation of five repeated measurements. (d) Fluorescence lifetime (τ) decays of Rh6G in an aqueous micellar solution with nitrite and without nitrite, respectively.
The nitrite and KI coexist leading to the fluorescence quenching of Rh6G, which can be expressed by the following chemical equation [25]:
Firstly, the iodide ion reacts with nitrite to generate iodine under acidity conditions. Next, the iodine reacts with excess iodide ions to form $\textrm{I}_{3}^{-}$. Finally, the $\textrm{I}_{3}^{-}$ will generate an association complex with Rh6G, resulting in the fluorescence of Rh6G being quenched by the heavy atom effect (as shown in Fig. 1(b)).
To verify the proposes of heavy atom effect, the fluorescence lifetime (τ) of Rh6G in the absence and presence of nitrite are measured, respectively. As shown in Fig. 3(d), the fluorescence lifetime of Rh6G would consistently be significantly decayed upon the addition of nitrite [26]. The decay lifetime curves can be fitted by an exponential formula [27]:
where τ is the component of the fluorescence lifetime. α1 and α0 are the corresponding fitting parameter and a constant, respectively. According to the results of the fitting, the fluorescence lifetime dropped from 6.45 ns to 3.25 ns after adding the nitrite. It means that the heavy atom effect imparts Rh6G with significantly shortened fluorescence lifetime due to the presence of nitrite.Moreover, to study the structure geometry optimization and electronic properties of Rh6G, the DFT calculations using Gaussian 09 programs have been performed. The electron distribution of the HOMO and LUMO in the optimized structure of Rh6G and the associative complex of Rh6G are evaluated through DFT calculations by using the B3LYP functions [28]. As shown in Fig. 4, the C = N on the xanthene ring of the rhodamine probe is opened, and C–I, and N–I are formed at the same time after coordinating with $\textrm{I}_{3}^{-}$. This interaction of intermolecular might interfere with the electron transition from the exited state back to the ground state, which leads to lasing emission decay and turn-off. This result solidified to support the above sensing mechanism.
Fig. 4. The electronic configuration in HOMO and LUMO levels of Rh6G and complex (Rh6G with nitrite). Illustrated atoms: red-O, blue-N, grey-C, white-H, pink-I.
The lasing intensity of Rh6G is a function of the nitrite concentration. Therefore, to estimate the sensing performance, the decrease in lasing intensity due to the binding of Rh6G with nitrite to form a non-fluorescence complex could be employed in estimating the strength and nature of the interaction. The following Stern−Volmer equation is used to calculate the quenching constant KD, which is described by [29]:
where I is the lasing intensities of the Rh6G after adding nitrite of different concentrations. I0 is the lasing intensities of the Rh6G without the quencher. [Q] represents the various concentrations of the quencher. The quencher in the present study is nitrite.To acquire the Stern−Volmer quenching constant KD, the I0/I of lasing versus the concentration of nitrite is fitted, as shown in Fig. 5. The response of the I0/I to the nitrite concentration is linear when the nitrite concentration increases from 5 to 200 µM. The correlative coefficient R2 = 0.994, as shown by the pink spheres in Fig. 5. In addition, the LOD is calculated with the formula LOD = 3σ/KD, here σ is the standard deviation of the blank solution (Rh6G alone) [30]. The LOD concentration of the proposed sensor is estimated to be approximately 0.54 µΜ, which is considerably lower than the maximum level (concentration of 65.2 µΜ) in drinking water set by the World Health Organization (WHO) [31].
Fig. 5. Response curve for both lasing and fluorescence at various nitrite concentration. Error bars represent standard deviation of five repeated measurements.
We also test the fluorescence intensity of Rh6G after adding different nitrite concentrations as a comparison. A continuous-wave (CW) laser (wavelength of 532 nm) is used as the exciting source. The experimental results show a relationship between the I0/I of fluorescence and the nitrite concentration is linear as the nitrite concentration increases from 10 to 500 µM, as shown by the red spheres in Fig. 5. The correlative coefficient R2 = 0.992. The LOD is calculated to be approximately 66.4 µΜ. The LOD of the lasing-based sensing is remarkably enhanced by nearly two orders of magnitude to that of fluorescence. Those data analyses illustrate that a FOFL sensor based on Rh6G in an aqueous micellar solution is highly competitive in sensitivity.
3.3. Selectivity of the $\textrm{NO}_{2}^{-}$ sensor
In addition to high sensitivity, the specificity and selectivity of the sensing target is another crucial attribute of the sensor. To assess the specificity of the FOFL system, the characteristics of the lasing of the Rh6G are investigated in the presence of other ions (Cl−, I−, Br−, $\textrm{CO}_{3}^{2-}$, $\textrm{SO}_{4}^{2-}$, CH3COO−, $\textrm{NO}_{3}^{-}$, and Na+) at the same concentration. Figure 6 shows the changes in the intensity of the lasing spectra integrate intensity. The lasing intensity of the R6G dye solution is not significantly affected by the presence of other coexisting ions except when nitrite ion is added. These results indicate that the selectivity of the FOFL systems for nitrite ions cannot be subjected to interference by other ions. Therefore, the FOFL system is suitable for the detection of nitrite ions.
Fig. 6. Plots of the normalized lasing intensity of Rh6G various coexisting ions. Pink bars plot the lasing intensity of Rh6G in the presence of coexisting ions. Green bars plot the lasing intensity of Rh6G in the presence of various coexisting ions after the addition of $\textrm{NO}_{2}^{-}$. Error bars are standard deviation of five repeated measurements.
Since sensors need to be applied in complex and realistic environments, the ability to avoid the interference of molecules other than the target is an important attribute. To investigate the effectiveness of the proposed FOFL sensor against possible disturbances that appeared in more complex and diverse conditions, we perform three additional control trials to study the anti-interference of the proposed FOFL sensor.
We compare the lasing response characteristics of 0.5 mM Rh6G in an aqueous micellar solution with/without the interference of other ions under the same pump energy density of 25.0 µJ/mm2. Figure 7 shows the detailed results. The lasing spectra of Rh6G with the interference of different contents are recorded by a 2400 g/mm grating, as characterized in Fig. 7(a). Moreover, the inset of Fig. 7(a) displays the lasing emission in operation with different interference contents. The lasing intensity of Rh6G, upon the addition of the interference ions (except nitrite) (II), is slightly influenced (as presented by the pink column graph in Fig. 7(b)). Therefore, the lasing threshold of the solution with the interference ions (without nitrite) added is very close to that of mixed of acidized Rh6G and KI (I) in an aqueous micellar solution (1.2 µJ/mm2), as shown in Fig. 7(c). However, the lasing spectrum does not have any mode structure and weak fluorescence emission (as shown in the inset) when adding the nitrite (III). The maximal lasing intensity attenuation factor is about 40-fold (as shown by the blue column graph in Fig. 7(b)). The Rh6G aqueous micellar solution undergoes a color transition from bright to dark upon the addition of interfering ions and nitrite (as illustrated in the inset of Fig. 7(b)). The specificity testing of the proposed FOFL sensor indicates its excellent selectivity for $\textrm{NO}_{2}^{-}$ under the presence of environmentally relevant competing ions. Therefore, Rh6G-based FOFL in an aqueous micellar solution has a potential application in detecting nitrite at low sample consumption.
Fig. 7. The response of lasing properties of 0.5 mM Rh6G in an aqueous micellar solution under various interfering conditions. (a) Comparison of the lasing spectra of Rh6G (I) (red curve), Rh6G with interfering ions (II) (pink curve), and Rh6G with interfering ions and nitrite (III) (blue curve) in an aqueous micellar solution. The PED is kept at 25 µJ/mm2. The insets show the images of the WGM lasing emission under different interfering conditions. (b) The spectra normalized intensity of Rh6G (red column graph), Rh6G with interfering ions (pink column graph), and Rh6G with interfering ions and nitrite (blue column graph), respectively. The insets display the Rh6G aqueous micellar solution undergoes a color transition from bright to dark with different interfering conditions. (c) Spectrally integrated output intensity as a function of PEDs. The error bars are standard deviation of five repeated measurements.
In addition, the comparison between the proposed WGM-based FOFL sensors and other frequently used nitrite sensors using different sensing probes is shown in Table 1. Compared with the fluorescent methods, the proposed FOFL has a certain advantage in the LOD. Especially, Due to the small size of the microfluid channel, the FOFL sensor only consumes a small analyte volume and leads to a low cost per test. Simultaneously, the sample solution is introduced into the microfluid channel through the liquid inlet without immobilizing the probe molecules on the surface of the optical fiber, which avoids the issue that the fiber cannot be recycled due to the immobilizing the probe molecules on the surface of the fiber in other sensors. In addition, the FOFL sensor can be further investigated for the detection of hazardous, toxic, and volatile samples. Therefore, the FOFL sensor is competitive as nitrite sensors.
Table 1. Comparison of different types of nitrite sensors.
3.4. Detection of nitrite and recovery studies in real samples
To verify the applicability of the proposed sensor used for nitrite sensing in real samples, tap water and DI water are utilized as representative samples for recovery studies using the developed sensor. To do this, all samples are spiked with a known concentration of nitrite. The testing results are shown in Table 2. The measured nitrite concentrations using the developed method are in good agreement with the standard concentrations, the recoveries of nitrite vary between 97.72% and 103.44%. Moreover, the recoveries are obtained with a relative standard deviation (RSD) of less than 5.0%. Nitrite ions are detected in real water samples using the proposed sensor, indicating that the proposed sensor has good accuracy and the analytical response is not influenced by the coexistence of other ions and species.
Table 2. Analytical results of nitrite in real samples (n = 5).
4. Conclusion
In summary, we demonstrate a chemosensor based on a WGM-based fiber optofluidic laser to monitor and detect nitrite ions, which is based on the heavy atom effect mechanism of Rh6G and nitrite ions. The concentration of the nitrite ions in the aqueous environment is realized in situ by real-time recording of the lasing intensity. Due to the intrinsic high-quality feedback structure of the FOFL system, resulting in the LOD is as low as 0.54 µM. The LOD of lasing emission based on the FOFL is more than 100-fold that in the fluorescent method. Moreover, the selectivity of the FOFL sensor is studied by using a variety of ions, and the testing results indicate that the sensor-based FOFL has an evident selectivity of nitrite ions. The mechanism of interaction between the Rh6G and nitrite ions is determined through DFT theoretical calculations. Considering the richness of fluorescent dyes and the different types of new sensing materials being developed, the proposed FOFL sensor will be expanded to the detection of hazardous, toxic, and volatile solutions. Additionally, the PDMS chip is not suitable for the detection of highly corrosive chemicals.
Funding
National Natural Science Foundation of China (11864045, 61965016, 62175209, 62241506); the Yunnan Fundamental Research Projects (202301BF070001-002).
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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