Introduction

Fluorescent anion sensors have vital applications in food safety, biological monitoring, biomedicine, and other areas. Among the common anions, F− performed a crucial role in industry, biology, and human daily life due to its smallest atomic radius, largest electronegativity, and strong Lewis basic properties [1]. For example, not only could it be used as an ingredient in toothpaste to treat osteoporosis, but it could also be used to purify uranium for nuclear weapons production. However, fluorine contamination was inevitably produced during the production processes, which disrupted many cell signaling processes and impeded the production of biological materials [2, 3]. Long-term consumption of high-fluorine water, food, and air by residents in high-fluorine areas would induce related diseases, such as fluorosis of the bones, cancer, neurological diseases, and so on. In addition, the degradation of some minerals and rocks on the slopes could also lead to an increase in fluoride ions in the aquatic environment, resulting in endemic fluorosis [4]. Therefore, it was crucial to create an effective and accurate procedure for F− detection. In 1996, Kimura discovered that H2S may be an endogenous neuromodulator present in the human brain [5, 6], which plays a significant role in the circulatory, respiratory, digestive, and central neurological systems [7,8,9]. H2S was discovered in a variety of meals and was produced spontaneously by organic sulfur-rich foods such as meat, eggs, fruits, and vegetables [10]. The research showed that appropriate, low-concentration H2S had a positive effect on food. H2S, for instance, could increase the metabolic rate of the antioxidant enzymes, thus extending the time that food could be stored [11]. Nevertheless, H2S in industrial wastes might harm mucous membranes, impair olfaction, and even kill people [12]. H2S detection techniques have drawn a lot of discussion in recent years. The fluorescence detection approach was more effective at detecting H2S than previous techniques due to its high sensitivity, non-invasive nature, ease of use, and other benefits [13,14,15,16,17,18]. As a result, it was essential to develop a new, accurate, and straightforward approach for H2S analysis and detection in order to increase food security.

Intensity-varying fluorescent probes typically had a single emission peak and were prone to interference from ambient factors, instrument parameters, and light bleaching during the detection process. This could lead to inaccurate detections, so they were unsuitable for cell imaging and food safety testing [19, 20]. In comparison to the switch model, the proportional sensing approach may provide an acceptable solution to the above problems because it could perform accurate analysis and testing by collecting the intensity ratio of the fluorescence probe’s two emission peaks and its unique response. However, a substantial emission peak displacement was usually required to obtain good ratio measurement performance, as this could well divide the two simultaneous emission peaks and prevent luminescence spectra from overlapping [21,22,23]. Therefore, the presence of near-infrared (NIR) fluorescent probes was deemed more favorable because of their greater penetration, lower autofluorescence background, and less photodamage to biological materials [24,25,26,27,28,29,30]. Hence, the creation of ratiometric fluorescent probes possessing features of near-infrared emission has long been the center of our attention.

Benzothiazole and its derivatives play a significant role in fluorescent probes, drugs, functional molecules, dyes, sensors, and so on due to their strong rigid structure, robust optical support, and therapeutic efficacy. Additionally, benzothiazole derivatives also possessed an excellent quantum yield and an immense Stokes shift, so benzothiazole derivatives had been widely used by researchers to construct fluorescent probes [31,32,33,34,35].

F− and H2S were two important small molecules, but there were few probes that had a recognition response to both F− and H2S. Based on this, a new tetraphenylethylene and iminocoumarin-based naked-eye colorimetric fluorescent probe, TPICBT, was successfully synthesized by Knoevenagel condensation reaction in this work, which could realize dual recognition of F− and H2S (Scheme 1). TPICBT could detect F− and H2S in DMSO solution in a ratio-type, respectively, with a large red shift greater than 90 nm, and was already successfully utilized on test paper strips. In addition, the probe displayed strong cell-membrane penetration and may be used for identifying H2S in active cells.

Scheme 1
scheme 1

The mechanism of TPICBT identification of F− and H2S

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Experimental Section

General Methods

The agents were presented in the supporting information ESI.

Synthesis of TPICBT

The synthesis methods of Compound 1 and Compound 2 were shown in Schemes S1 and S2, and the relevant structural characterizations were listed in Figs. S1-S4. The synthesis route of TPICBT was shown in Scheme 2. After dissolving Compound 2 (0.376 g, 1 mmol) and benzothiazol-2-acetonitrile (0.174 g, 1 mmol) in anhydrous ethanol, they were agitated for 0.5 h at room temperature in argon. Then piperidine was dropped and heated under an argon atmosphere for reflux for 5 h to obtain the crude product. Rinse with anhydrous ethanol to finally obtain compound TPICBT (0.315 g, 83.8%). 1H NMR (500 MHz, DMSO-d6) δ 9.06 (s, 1H), 8.65 (s, 1H), 8.13 (d, J = 5.0 Hz, 1H), 8.04 (d, J = 10.0 Hz, 1H), 7.60 (d, J = 5.0 Hz, 1H), 7.54 (t, J = 7.5 Hz, 1H), 7.44 (t, J = 7.5 Hz, 1H), 7.21–7.14 (m, 9H), 7.07–7.03 (m, 4H), 6.99 (d, J = 5.0 Hz, 2H), 6.85 (d, J = 10.0 Hz, 1H), 6.75 (s, 1H). 13C NMR (126 MHz, CDCl3) δ 156.4, 151.8, 147.1, 146.7, 142.5, 142.0, 142.0, 141.8, 138.4, 134.4, 132.5, 130.3, 130.2, 130.2, 127.0, 127.0, 126.8, 126.7, 126.6, 126.2, 125.9, 125.9, 122.1, 121.6, 118.1, 116.9, 116.6, 116.0, 110.6. ESI-MS (m/z): calcd for C36H24N2OS: 533.1606 [TPICBT + H]+, found: 533.1676 (Fig. S5-S7).

Scheme 2
scheme 2

Synthetic route of TPICBT

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Results and Discussion

Optical Properties of Fluorescent Probe TPICBT

The photophysical properties of TPICBT in the free state (pure THF solution) and the concentrated state (solid state) were studied through ultraviolet absorption spectroscopy and fluorescence spectroscopy (C = 2.0 × 10− 5 mol L− 1, λex = 390 nm, slide = 5 nm / 5 nm), the fundamental spectral parameters shown in Fig. 1, and the basic spectral parameters were shown in Table 1. In pure THF solution, TPICBT had two absorption peaks at 323 nm (ɛ = 1.66 × 104 L mol− 1 cm− 1) and 410 nm (ɛ = 4.23 × 104 L mol− 1 cm− 1). The π-π* transition was responsible for the weak peak at 323 nm, while the formation of the strong absorption peak at 410 nm was primarily driven by the intramolecular charge transfer (ICT) of electrons between TPE and benzimidazole, which act as electron donors and acceptors, respectively [36]. In the solid state, TPICBT’s absorption peak was primarily at 341 nm (ɛ = 5.16 × 104 L mol− 1 cm− 1). Furthermore, as shown in Fig. 1A, when the λex was 390 nm, the fluorescence spectrum of TPICBT in pure THF solution exhibited yellow-green fluorescence at 545 nm, and there was a large Stokes shift (Δλ = 135 nm). It showed that TPICBT has low background interference and strong sample penetration, which could be better applied in cell imaging. In the solid state, the fluorescence emission peak of TPICBT displayed a bright yellow-green luminescence and was measured at 554 nm. Compared with TPICBT in pure THF solution, the solid emission peak showed a redshift of 9 nm and exhibited a larger Stokes shift (Δλ = 213 nm). The variance between the two could be attributed to the variable fluorescent molecule accumulation patterns and fluorescent molecule interactions in the free-concentrated state [37]. In addition, the fluorescence decay curve in the substance state was also displayed in Fig. 1B, where a double exponential function value (τ) = 1.30 ns was calculated. Compared with the fluorescence lifetime index in the free state (τ) = 1.05 ns, this result further proved that TPICBT in the solid state could release energy in a fluorescent way due to molecular aggregation in solids and limited molecular motion [38, 39]. The comparison between the solid state and the free state showed that TPICBT had the properties of AIE.

Table 1 Optical characterization of TPICBT in THF and solid state
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Fig. 1
figure 1

(A) The absorption and fluorescence spectra of TPICBT in the free state (pure THF solution) and the concentrated state (solid state), and (B) fluorescence decay curve of TPICBT in the solid state

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Aggregation-induced Emission (AIE) Properties of the Fluorescent Probe TPICBT

To further investigate the likely AIE characteristics of TPICBT, the UV-vis absorption and fluorescence spectra in the THF/H2O mixture were studied at room temperature. Due to the formation of nanoaggregates and the Mie effect, the TPICBT’s absorption spectra, as displayed in Fig. 2A, had nearly identical absorption, and a noticeable level-off tail emerged in the long wavelength area [39]. In the fluorescence spectrum, it could be seen that TPICBT was incredibly low in pure THF, and the fluorescence yield of the quantum (Φf) was just 0.015. However, when the fw gradually increases, the fluorescence intensity also gradually grows, and an abrupt increase in emission intensity was observed when the f w was above 90%. And when the fw was at 99%, the Φf of the solution was 0.058, which was about four times higher than that of pure THF. Additionally, when exposed to a 365 nm UV portable lamp, the fluorescence color of TPICBT could be found to change from yellow-green to bright yellow (inset: Fig. 2B). These occurrences might have resulted from TPICBT’s creation of nano-aggregates when the system’s fw was raised, which triggered the limitation of the intramolecular rotation (RIR) effect [40], thereby causing the high emission of the THF/H2O mixture. The above results showed that TPICBT had a typical AIE effect.

Fig. 2
figure 2

(A) The absorption and fluorescence spectra of TPICBT (λex = 390 nm) in THF/H2O mixtures (C = 2.0 × 10− 5 mol L− 1) with different fw, and (B) the fluorescence intensities of TPICBT in different fw at 560 nm (inset: TPICBT photos taken under 365 nm UV light)

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Furthermore, since the fluorescence lifetime measurement was uncorrelated with the chromophore concentration and intensity of excitation, it could better illustrate the AIE characteristics of TPICBT. At a water percentage of 0% and 99%, the fluorescence lifetimes of TPICBT were 1.05 ns and 1.18 ns, respectively, as shown in Fig. 3A. Following that, Eqs. (1) and (2) were used to compute the average lifetime (τ) and quantum yield (Φ) in order to get the radiative rate constant (kr) and non-radiative rate constant (knr). The calculated findings revealed that the kr value grew from 0.014 ns− 1 (fw = 0%) to 0.049 ns− 1 (fw = 99%), whereas the knr value fell from 0.938 ns− 1 to 0.797 ns− 1. The fluorescence enhancement was triggered by a rise in radiative rate constants and a fall in non-radiative rate constants. The results of fw = 0% and fw = 99% were consistent with the above conclusions. Further evidence from these findings pointed to the fact that constrained intramolecular rotation in aggregates impedes the non-radiative decay process and increases fluorescence.

$${{\rm{\tau }}^{{\rm{ - 1}}}}{\rm{ = }}{{\rm{k}}_{\rm{r}}}{\rm{ + }}{{\rm{k}}_{{\rm{nr}}}}$$
(1)
$${{\rm{k}}_{\rm{r}}}{\rm{ = \phi/\tau }}$$
(2)

Finally, we also carried out the DLS experiment in a mixed solution (fw = 99%) to investigate the size of TPICBT. As shown in Fig. 3B, the average size of TPICBT nanoaggregates was 120 nm. This result indicated that TPICBT has the ability to self-assemble into nano-agglomerates in an aqueous solution, which strongly supports the material’s AIE characteristics.

Fig. 3
figure 3

(A) Fluorescence decay curve of TPICBT in THF/H2O (fw = 0, 99%, C = 2.0 × 10− 5 mol L− 1) solution; (B) dynamic light scattering of TPICBT (fw = 99%)

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Selectivity and Anti-interference Ability of the Fluorescent Probe TPICBT

Firstly, in order to explore the selectivity of TPICBT for F− and H2S, the interaction between TPICBT and various anions was analyzed, including F−, HS−, CO32−, SCN−, HCO3−, HSO3−, SO42−, Cl−, NO3−, I−, H2PO4−, HSO4−, Br−, NO2−, and S2O32−. As depicted in Fig. 4A, a peak at 388 nm was seen in the UV absorption spectrum of the probe TPICBT. However, until F− was introduced, the initial absorption peak at 388 nm dropped, and a new peak appeared at 570 nm. Under sunlight, the color of visible light could be observed to range from yellow to bluish-purple (Fig. 4C). Besides, TPICBT would exhibit a novel absorption peak at 553 nm, and visible light would turn lavender instead of yellow in the presence of H2S (Fig. 4C). But it could be found that the UV absorption wavelength of probe TPICBT combined with F− was redshifted by 17 nm compared with H2S. Moreover, the visible color difference between F− and H2S identified by TPICBT could be clearly distinguished with the naked eye. Therefore, this could be used as a standard to distinguish the two.

Fig. 4
figure 4

(A) Absorption spectra; (B) fluorescence spectra of TPICBT (C = 2.0 × 10− 5 mol L− 1) in the presence of contending anions; (C) comparison plot of TPICBT recognizing F− and H2S

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Then, as shown in Fig. 4B, when it was excited at 390 nm, the original fluorescence emission peak at 557 nm both disappeared after the addition of F− and H2S. Additionally, new emission peaks appeared at 657 nm and 653 nm in the near infrared, respectively, with a redshift of about 100 nm. And the fluorescence color changed from yellow to deep red and dark red when exposed to UV light (Fig. 4C). However, upon the addition of more ions, the TPICBT solution remained mostly unchanged (Fig. S8). These results showed that the probe TPICBT could achieve specific recognition of F− and H2S. In addition, we further assessed the anti-interference capabilities of TPICBT. As shown in Fig. S9A, when other ions were added to the solution for simultaneous existence with F−, the fluorescence intensity did not change significantly. When other ions co-existed with H2S, only the presence of F− would enhance the fluorescence intensity as a whole, while the purity of other ions had no significant effect on the fluorescence intensity of Fig. S9B. The results showed that the probe had good anti-interference ability. Therefore, TPICBT was an outstanding near-infrared fluorescence probe for F− and H2S detection.

Concentration Response and Detection Limit of Probe TPICBT on F−

Moreover, to assess the dose-dependent response of TPICBT, the fluorescence and UV-vis response of TPICBT to F− were measured by concentration titration at room temperature. When 0-2.6 eq. F− were gradually added to the TPICBT solution, the absorption peak gradually decreased at 398 nm and increased at 562 nm (Fig. S10A). As shown in Fig. S10B, a wonderful linearity between absorbance ratios (A562/A398) and F− concentration was acquired in the range of 1-4.5 µM (Y = 0.23881*X - 0.15517, R = 0.99452). At the same time, with the addition of F−, the fluorescence peak at 657 nm gradually increased, while that at 557 nm gradually decreased (Fig. 5A). A clear isoborption point was observed at 620 nm, and the fluorescence quantum yield increased from 0.020 (TPICBT) to 0.042 (TPICBT + F− complex), indicating that the probe TPICBT reacted with F−. In addition, the radiation ratio (I657/I557), an important feature of the ratio response, increased from 0.41 to 13.6. In the range of 0.5-3.0 M, the emission ratio (I657/I557) versus the F− concentration displayed an excellent linear correlation (Y = 0.513*X + 0.057, R = 0.97106) (Fig. 5B), indicating that the probe TPICBT may be able to accurately quantify F−. Based on the reported mean value, the detection limit of TPICBT was computed to be 6.01 µΜ, which was far less than the World Health Organization’s maximum pollutant level for F− in drinking water (1.5 mg L− 1, about 80 mM) [41,42,43]. These findings demonstrated the potential of the TPICBT probe to be a ratio metric fluorescence probe for the visual identification of F−, offering a quick and practical way to detect F−.

Fig. 5
figure 5

(A) The fluorescence variation of TPICBT (C = 2.0 × 10− 5 mol L− 1) induced by F−, and (B) the linear relationship between the ratio of two fluorescence intensities (I657/I557) and F− concentration in DMSO solution

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Detection Mechanism of Fluorescent Probe TPICBT for F−

Additional evidence of the probe TPICBT’s binding to F− in DMSO-d6 was provided by the 1H NMR study (Fig. S12). The 1H NMR contrast map showed that the chemical signal belonging to hydrogen on = NH, which was placed at 9.08 ppm, would gradually weaken and migrate to 9.05 ppm with the addition of F−, but it did not fully disappear. The reason why the = NH proton did not deprotonate was caused by the extremely electronegative F− engaging with it to form a hydrogen bond [44, 45]. Additionally, the maximum emission intensity of the [TPICBT] + [F−] complex at 657 nm was seen when the mole fraction of [TPICBT]/([TPICBT] + [F−]) was around 0.5, demonstrating a 1:1 stoichiometric ratio between TPICBT and F− (Fig. S11). Finally, mass spectrometry was used to confirm the mechanism even further. The mass spectrum changes were depicted in Fig. 6, and the [TPICBT + H+]+ mass spectrum signal at m/z = 533.1676 increased to 552.1620 ([TPICBT + F− + H+]+) (calcd. 552.1593) with the addition of F−. These indicated that F bound to H in = NH and the deprotonation process did not occur.

Fig. 6
figure 6

Comparison of mass spectra of [TPICBT + H+] + and [TPICBT + F−+ H+]+

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Concentration Response and Detection Limit of Probe TPICBT on H2S

According to previous literature, when an alkene C = C was coupled to an electron-withdrawing group, a Michael addition reaction involving nucleophilic mercaptans could directly start the reaction [46, 47]. Therefore, we conjectured that TPICBT could recognize H2S by binding to C = C bonds. In order to test the recognition sensitivity of TPICBT to H2S, concentration titration experiments were also performed. Upon gradually adding H2S, a new peak at 548 nm and an isosbestic point at 450 nm appeared when the absorption intensity at 389 nm decreased. The absorption spectrum reached equilibrium following the addition of 8.0 eq. H2S (Fig. S13A). It was discovered that the absorption intensity ratio (A548/A389) and the H2S content had a linear connection in the 0–16 µM range (Y = 0.11785*X - 0.24479, R = 0.98272) (Fig. S13B). Additionally, Fig. 7A displays the TPICBT’s fluorescence spectra in reactions with various H2S concentrations. The fluorescence emission peak of TPICBT at 557 nm progressively declines as H2S is introduced, whereas it gradually increases at 653 nm, and an isorption point develops at 619 nm. Moreover, the emission ratio (I653/I557) against the H2S concentration in the region of 6–15 µM displayed an excellent linear relationship (Y = 0.341*X - 1.536, R = 0.98412) (Fig. 7B). Using the equation LOD = 3σ/k, the detection limit of TPICBT toward H2S was calculated to be 8.68 µM based on the slope of the curve and the standard deviation of the blank solution. The detection limit of the TPICBT probe was lower than the nationally permissible upper concentration of H2S in the workplace of 10 ppm (15 mg/m3) [48]. The above results showed that TPICBT was a sensitive H2S detector.

Fig. 7
figure 7

(A) The fluorescence variation of TPICBT (C = 2.0 × 10− 5 mol L− 1) induced by H2S; (B) the linear relationship between the ratio of two fluorescence intensities (I653/I557) and H2S concentration in DMSO solution

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Detection Mechanism of Fluorescent Probe TPICBT for H2S

In order to learn more about the chemical process between probe TPICBT and H2S, 1H NMR and mass spectra of the probe TPICBT were measured. Upon the addition of H2S, the chemical shift at 8.11 ppm (Ha) disappeared due to H2S attacking the C = C, while new signal peaks appeared at 4.38 ppm (C-H, Hb), 3.16 ppm (C-H, Hc), and 1.56 ppm (-SH) (Fig. S14). These results demonstrated that H2S nucleophilic addition attacked the alkenyl group [49], which changed the fluorescence of the probe. Lastly, mass spectrometry was used to confirm the mechanism even further. The mass spectrum changes were depicted in Fig. 8, and the [TPICBT + H+]+ mass spectrum signal at m/z = 533.1676 increased to 567.3679 ([TPICBT + HS− + H+]+) (calcd. 567.3679) with the addition of HS−.

Fig. 8
figure 8

Comparison of mass spectra of [TPICBT + H+] + and [TPICBT + HS−+ H+]+

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Table S1 listed some of the fluorescent probes that had been reported for the recognition of F− and H2S [50,51,52,53,54,55,56,57,58,59]. It was found that most of the fluorescent probes using TPE as a parent were almost off-on-type probes, rarely involving ratiometric-type probes. In comparison, the ratiometric probe was more accurate than the off-on probe, and these probes could not detect both F− and H2S simultaneously. In this work, TPICBT fluorescent probes could not only realize F− and H2S in ratiometric recognition but also the fluorescence emission peak at more than 650 nm. In the near-infrared region, it could be observed that the large redshifts (˃ 90 nm) and near-infrared probe could effectively reduce the interference of background, which was a good advantage in detection.

Practical Application of the Fluorescent Probe TPICBT

To evaluate the analytical application potential of TPICBT probes, filter paper was soaked in a DMSO solution containing TPICBT, dried in air, and deionized aqueous solutions of various ions were dropped on the test strip. As shown in Fig. 9, the test strip with other ions added did not change appreciably, but when F− was added, the apparent color changed from light yellow to brilliant purple, as well as its fluorescent color switching from yellow to bright red. In addition, compared with the test strip for detecting F−, the H2S test strip’s visible light color changed to dark purple, and the fluorescence color changed to dark red. The above phenomenon was similar to the fluorescence and ultraviolet test comparison diagram in Fig S8, and could be used as a proof to distinguish F− and H2S. According to these findings, the probe TPICBT could be utilized as a portable solid-state device for F− and H2S detection.

Fig. 9
figure 9

Fluorescence and visible color change of different anions added to test strips, sunlight (up), and a 365 nm UV lamp (down)

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One of the primary volatile ingredients in the degradation of raw food was thought to be H2S, a sulfur-containing bacterial metabolite. High H2S residual concentrations from spoiled food could have negative health consequences for people [11, 60,61,62,63]. Therefore, the response experiment of the probe to the spoilage degree of raw meat was carried out. In this experiment, the color and fluorescence changes of the original meat sample strips were recorded. In the blank control group (Fig. S15), because the raw meat was stored at 0 ℃, it did not deteriorate and produce H2S within 72 h, so the fluorescence and visible color of the test strip did not change within 72 h. According to Fig. 10, when raw beef was exposed to room temperature for 72 h, it partially decomposed and created H2S, changing the color of the test paper from yellow to light purple and the fluorescence from bright yellow to light red. It showed that the test paper could sensitively detect H2S gas emitted from rotten raw meat. The above results showed that TPICBT had a specific response to H2S and might be applied as a method to evaluate raw meat freshness using simple dipstick assays.

Fig. 10
figure 10

At room temperature, photos of probe TPICBT detecting H2S gas emitted from raw meat at different time periods: (A) sunlight; (B) a 365 nm UV lamp

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Cell Imaging of the Fluorescent Probe TPICBT

To further evaluate whether TPICBT has practical application in organisms, we identified the cytotoxicity of TPICBT using the MTT method and observed whether fluorescence imaging of H2S could be performed in HeLa cells. It was evident that after being exposed to TPICBT at doses of 0–20 µmol for 24 h, HeLa cells maintained at least 85% of their metabolic viability. In accordance with the findings, TPICBT exhibited high biocompatibility and low cytotoxicity (Fig. S16). Following that, the confocal laser scanning microscope (CLSM) was used to observe the ability of TPICBT to identify exogenous H2S in HeLa cells (λex = 390 nm). The uniform distribution of the green fluorescence signal in the cells (Fig. 11B2) indicates that TPICBT successfully penetrated the cell membrane of HeLa cells. After the cells had been exposed to TPICBT (20 µM) and NaHS (50 µM) for 30 min, the cells thereafter displayed red fluorescence (Fig. 11C2). The outcomes demonstrated the TPICBT probe’s potential for imaging H2S in living cells.

Fig. 11
figure 11

Cell imaging of H2S recognition by probe TPICBT, blank (A1, A2, A3), TPICBT (B1, B2, B3), and TPICBT + H2S (C1, C2, C3)

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Conclusions

In summary, based on tetraphenylethylene and iminocoumarin, we have effectively designed and synthesized the probe TPICBT with high selectivity, a low detection limit, and a fast response. TPICBT had an obvious AIE effect with a relative redshift greater than 90 nm and could be used as a proportional near-infrared fluorescence probe for rapid detection of F− and H2S. In real life, the test strips made up of TPICBT probes could be used to quickly detect F− and H2S with the naked eye. In addition, TPICBT could sensitively detect the release of H2S in raw meat, which could be used to detect the freshness of food. Moreover, because TPICBT has good cell permeability and is not hazardous to living things, it may be utilized as a fluorescent dye for cell fluorescence imaging. We think that this study’s logical design approach could serve as an inspiration for the creation of more potent dual-responsive fluorescence probes for F−/H2S and other biomolecules.