Long‐lasting chemiluminescence by aggregation‐induced emission surfactant with ultralow critical micelle concentration

The development of green and simple chemiluminescence (CL) systems with intensive and long‐lasting emission is highly desirable in lighting and extension of their applications. In this study, it is found that the involvement of aggregation‐induced emission (AIE) surfactant could greatly enhance the CL of luminol–H2O2–Co2+ system. The inserted hydrophobic tetraphenylethylene fluorophore in AIE is able to increase the hydrophobicity of alkyl chain and decrease the critical micelle concentration (CMC) of surfactant. The synergistic effect of micelle‐improved enrichment and CL resonance energy transfer endows luminol–H2O2–Co2+ system intensive and long‐lasting emission under neutral pH conditions (pH 7.4). The visible emission is still observed even after 60 min. Our study has opened a new avenue for exploring green and simple effective CL systems through AIE surfactant with unltralow CMC toward various applications in lighting, optical sensing, and photocatalysis, etc.


INTRODUCTION
[11] In contrast, CL systems with strong intensity and glow-type emission are more charming to the development of cold light sources.To meet this concern, few CL systems with sustained-release of reactants have been developed via introducing hydrogel or tablet. [12,13]or example, Liu et al. reported intensive and long-lasting CL hydrogels by integrating chitosan, N-(4-aminobutyl)-N-ethylisoluminol and Co 2+ . [12]With the combination of luminol tablet and poly(vinylpyrrolidone)-H 2 O 2 complex solution, our group constructed a long-lasting (∼13 h) CL light using Tris-Co 2+ complex as the catalyst. [13]Nevertheless, the above fabricated systems were complicated, limiting their wide use.Therefore, it is highly necessary to explore a simple and long-lasting CL system.
It is reported that typical luminol-H 2 O 2 -Co 2+ system possesses relatively slow CL kinetics under neutral pH conditions. [14]Unfortunately, the ultraweak CL intensity of luminol-H 2 O 2 -Co 2+ system at neutral condition hinders its further application.[17] For instance, Xie et al. found that the CL intensity of peroxyoxalate derivativel-H 2 O 2 system can be greatly enhanced upon introducing cetyltrimethylammonium bromide (CTAB, a kind of typical cationic surfactant). [18]However, the previous reports demonstrated that the CL promotion was only implemented with high surfactant concentration, usually above critical micelle concentration (CMC), and CMCs of conventional surfactants are generally in mM level. [19,20]The requirement of high surfactant concentration is conflicting to the green chemistry and environment friendly.Therefore, the development of surfactant with low CMC is required in order to decrease its usage and side effect to environment.
CL resonance energy transfer (CRET) is usually utilized to further improve CL signals.In this case, additional fluorophores as energy acceptors are required in conventional surfactant-enhanced CL systems.For example, Song et al. reported the construction of intensive and long-lasting multicolor CL systems by introducing fluorescein or rhodamine B as the energy acceptor. [7]Nevertheless, the multicomponent involvement results in system complexity and weak stability.Accordingly, a direct linking of fluorophore into surfactant would make CL system simple.Hence, the introduction of novel surfactant with ultralow CMC and CRET acceptor is appealing for a simple and long-lasting CL system.
Stepanek et al. proposed fluorescent block copolymer surfactant by linking p-vinyl-9,10-diphenylanthracene (DPA) into polystyrene-block-poly(methacrylic acid). [21]Unfortunately, the aggregation-caused quenching (ACQ) properties of DPA led to weak fluorescence upon forming micelle.The ACQ features of the fabricated fluorescent block copolymer surfactant would restrict its potential applications in CL as the CRET acceptor.[31][32][33] Herein, we described our attempts to apply the developed AIE surfactants into long-lasting luminol-H 2 O 2 -Co 2+ CL system. [34]In a case study, we selected the TPE inserted dodecyltrimethylammonium bromide (4th cationic headgroup, denoted as C 8 -TPE-C 4 TAB) for the CL signal enhancement at pH 7.4.The CL intensity revealed 20-fold increment in the presence of only 80 μM C 8 -TPE-C 4 TAB, and glow-type emission appeared.The enhanced CL and long-lasting emission were due to the integration of micelle-improved enrichment, the diffusion control of reactants and high-efficient CRET between excited luminol and TPE (Figure 1).In other word, the CL reactants were enriched through electrostatic attraction, which subsequently benefits the CL reaction and the CRET process.The long-lasting CL was visible to naked eyes even after 60 min.The proposed fluorescent surfactant-assisted long-lasting emission showed advantages of convenient operation, low surfactant consumption and environment-friendly.The development of simple and green long-lasting CL systems for analytical sensing or lighting is possible by introducing novel fluorescent surfactants with ultralow CMCs.

C 8 -TPE-C 4 TAB-enhanced CL of luminol-Co 2+ -H 2 O 2 system
As a starting point for our study, we synthesized fluorescent surfactant C 8 -TPE-C 4 TAB according to our previously reported method and characterized it with proton nuclear magnetic resonance ( 1 H NMR), mass spectrum (MS), steadystate and time-resolved fluorescence spectrometry. [34]All results revealed the successful preparation of C 8 -TPE-C 4 TAB (Figures S1-S3).As is known, surfactants could facilitate the CL reaction and enhance the CL signals. [35,36]In this study, the CL signals of luminol-Co 2+ -H 2 O 2 in the absence and presence of C 8 -TPE-C 4 TAB were recorded under pH 7.4 conditions.As shown in Figure 2, the CL intensity was dramatically enhanced (∼20 folds) after the addition of 80 μM C 8 -TPE-C 4 TAB, indicating the feasibility of C 8 -TPE-C 4 TAB-improved CL intensity of luminol-Co 2+ -H 2 O 2 system.We have also tested the CL signals of Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system.It was found that no visible CL signals appeared (Figure S4), revealing that the CL background from TPE and CL reagents can be ignored.In addition, the reaction kinetics was also dramatically decelerated in the presence of C 8 -TPE-C 4 TAB.The enhanced intensity and extended CL emission kinetics indicated the possibility to construct long-lasting luminol-Co 2+ -H 2 O 2 system with the assistance of C 8 -TPE-C 4 TAB.

CL enhancement in the presence of C 8 -TPE-C 4 TAB or conventional cationic surfactant
As referred in previous work, [34] the CMC of C 8 -TPE-C 4 TAB was measured to be 46 μM.In this case, C 8 -TPE-C 4 TAB existed as the micelle but not monomer or dimer with 80 μM concentration.According to high resolution transmission electron microscope (HRTEM) measurement (Figure 3A), the formation of C 8 -TPE-C 4 TAB micelle (∼6.2 nm, 100 counts) was observed.These results demonstrated that the enhanced CL was attributed to the C 8 -TPE-C 4 TAB micelle-promoted CL reaction.To understand the differences of C 8 -TPE-C 4 TAB micelle and typical ternary ammonium micelle promoted CL, the CL signals of luminol-Co 2+ -H 2 O 2 system upon adding C 8 -TPE-C 4 TAB and CTAB were measured.The CMC of CTAB is reported to be 1.0 mM, and thus 2.0 mM CTAB was added to ensure the formation of CTAB micelle. [37]The HRTEM results indicated the formation of micelle with 2.0 mM CTAB (Figure S5).As shown in Figure 3B, the addition of CTAB largely enhanced the CL signals, indicating the universality of micelle-promoted CL.However, the CTAB-induced CL enhancement was lower than that of C 8 -TPE-C 4 TAB.Since the CTAB concentration is much higher than that of C 8 -TPE-C 4 TAB, the CTAB-induced CL enhancement may depend on its concentration.To understand the concentration effect, the CL signal of luminol-Co 2+ -H 2 O 2 system were recorded upon adding CTAB with the different concentrations.As manifested in Figure 3C, the CL signals gradually increased with an increase in CTAB concentration, and showed a sudden increment around CMC.Further higher concentrations didn't show dramatic increase of CL.Therefore, the CTAB concentration with two-folds of CMC would be enough to afford the stable micelle conditions.
In order to further figure out the possible reasons of the different CL enhancement characters, the other conventional cationic surfactants were tested, including tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide (DTAB) and decyltrimethylammonium bromide (DeTAB).To guarantee the formation of corresponding micelle, the concentrations of these surfactants were set to be two-folds of their CMCs. [38]The concentrations of TTAB, DTAB, and DeTAB were 8, 32, and 114 mM, respectively.It was seen that the micelle-improved CL signals increased with the increasing alkyl chain length of surfactants (Figure 3D).Upon the addition of cationic surfactants with short alkyl chain length, even with ultrahigh concentrations (>30 mM), the CL signals were much lower than that by adding C 8 -TPE-C 4 TAB (80 μM).These results suggested that C 8 -TPE-C 4 TAB was superior to conventional cationic surfactants on the CL improvement toward luminol-Co 2+ -H 2 O 2 system.

Mechanism of C 8 -TPE-C 4 TAB-mediated intensive and long-lasting CL emissions
As indicated above, the addition of C 8 -TPE-C 4 TAB endowed more effective CL reaction than conventional cationic surfactants.It is reported that cationic micelle can adsorb anionic reactants and benefit the CL reaction. [39,40] (+11.0 mV) because of the outside ternary ammonium cation (Figure S6), while luminol was negatively charged (−4.8 mV).[42][43] Such an electrostatic attraction-assisted CL emission was also proved by sodium dodecyl sulfonate (SDS, a kind of typical anionic surfactant).As shown in Figure S7, the CL intensity of luminol-Co 2+ -H 2 O 2 system distinctly decreased upon adding SDS.This is because the outside sulfonic acid groups make SDS micelle negatively charged, which possesses strong electrostatic repulsion toward luminol dianion and peroxide anion. [20]o further understand the mechanism of C 8 -TPE-C 4 TABenhanced CL intensity and long-lasting emission, a few characterizations were performed.The generation of reactive oxygen species (ROS) is important to the CL signals. [44]e tested the CL inhibition with the addition of different ROS scavengers.As displayed in Figure S8, p-benzoquinone (HQ) and thiourea caused visible CL suppression, while NaN 3 induced slight CL decrease, suggesting the primary ROS were superoxide radical (O 2 [47] The electron paramagnetic resonance (EPR) results also demonstrated the continuous generation of O 2 •− and • OH during the CL reaction.As shown in Figure 4, the sextuple peaks (∼2:2:1:2:1:2) located at 3477, 3487, 3492, 3502, 3506, and 3517 G values, are attributed to the characteristic O 2 •− EPR signals.Meanwhile, the characteristic • OH EPR signals with quadruple peaks (∼1:2:2:1) located at 3471, 3486, 3501, and 3516 G values were also observed.These results proved the continuous generation of O 2 •− and • OH.It is reported that many ROS possess longer lifetime in hydrophobic environment. [48]When inserting TPE into the backbone of surfactant in this work, the hydrophobicity of alkyl chain increased, this might benefit the stabilization of ROS and promote the CL reaction.
As mentioned in CTAB concentration study, the formation of micelle plays important role in enhancing the CL emission.Thus, the total micelle number would affect the CL reaction.That is, less micelle leads to the high concentrations of reactants on the surface of micelle, and promotes the CL emission.In contrast, the average surface concentrations of reactants decreased with the increasing total micelle number.In this case, the CL reaction was inhibited and only weak CL emission could be observed.For C 8 -TPE-C 4 TAB and CTAB improved luminol-Co 2+ -H 2 O 2 systems, the final concentration of C 8 -TPE-C 4 TAB and CTAB were 80 μM and 2.0 mM, respectively.The aggregation number of micelle was calculated to be 64 in CTAB micelle, while it became 36 in C 8 -TPE-C 4 TAB micelle. [49]The total micelle numbers of CTAB micelle and C 8 -TPE-C 4 TAB micelle were determined to be 3.8 × 10 15 and 2.3 × 10 14 , respectively.The CTAB micelle number was 16-folds higher than that of C 8 -TPE-C 4 TAB micelle.Obviously, a single C 8 -TPE-C 4 TAB micelle surface exist more luminol dianion and peroxide anion, which contributes the higher CL signals.Similarly, the aggregation numbers of DeTAB, DTAB, and TTAB micelle were calculated to be 39, 42, and 106, respectively.The corresponding total micelle numbers of DeTAB, DTAB, and TTAB micelle were determined to be 7.0 × 10 17 , 1.8 × 10 17 and 1.8 × 10 16 , respectively.It is clearly that the total micelle number of conventional surfactant distinctly increases with the decreasing alkyl chain length.In this case, the average surface luminol dianion and peroxide anion decrease with the decreasing alkyl chain length of surfactant, leading to the inhibited CL intensity (Figure 3D).
In addition to high enrichment effect, CRET is also needed be considered in the CL enhancement of C 8 -TPE-C 4 TAB.The TPE fluorophore of C 8 -TPE-C 4 TAB could act as the CRET acceptor, and its AIE characters could avoid the luminescence quenching during the formation of micelle. [50,51]In contrast, CTAB in the absence of intrinsic fluorophore could not act as the CRET acceptor.Then, the CL emission spectra of luminol-Co 2+ -H 2 O 2 system with the addition of CTAB or C 8 -TPE-C 4 TAB were obtained.As shown in Figure 5A, the emission maximum of luminol-Co 2+ -H 2 O 2 system located at 425 nm upon adding CTAB.The 425-nm light belongs to the excited luminol radicals. [52]However, the emission maximum shifted to 486 nm in the presence of C 8 -TPE-C 4 TAB (Figure 5B) and the CL emission spectrum was overlapped with the fluorescence emission profile of C 8 -TPE-C 4 TAB.The shifted emission maximum and characterized CL spectrum implied the occurrence of CRET in luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system. [53,54]To determine the CRET efficiency (φ), the integral areas of emission of TPE acceptor (denoted as I A ) and total system (denoted as I T ) were acquired by peak fitting analysis (Figure S9).Based on the equation: φ = I A / I T , the φ was calculated to be 98.5%. [34]s a result, the high enrichment and CRET contributed To understand whether the insert location affected the CL emission, the CL signals of luminol-Co 2+ -H 2 O 2 system upon adding C 4 -TPE-C 8 TAB (8th cationic headgroup insert), and TPE-C 12 TAB (12th cationic headgroup insert) were recorded.As shown in Figure S10A, the CL intensity decreased as the extended insert location.A possible reason was due to the decreased CRET efficiency because the distance between excited luminol and TPE increases with the extended insert location (Figure S10B).As indicated in the previous work, [34] the distance between TPE and excited luminol was determined to be 1.19 nm in C 8 -TPE-C 4 TAB-luminol system, while it became 1.73 nm in C 4 -TPE-C 8 TAB-luminol system, and 2.24 nm in TPE-C 12 TAB-luminol system, respectively.The CRET efficiencies in these three systems were calculated to be 99.14%,93.41%, and 72.39%, respectively.Thus, the rasied distance increased the energy loss and then decreased the CRET efficiency.Accordingly, all subsequent CL experiments were conducted with C 8 -TPE-C 4 TAB.
It is reported that the pH distribution in CTAB micelle solution is not uniform because of the adsorption of hydroxyl anions around the outside ternary ammonium cation layer. [20]herefore, the local pH around micelle surface is slightly higher than that in solution.As is known, luminol could emit the strong CL under alkaline environment. [55]To understand whether the surface adsorption hydroxyl anions affect the CL reaction, the CL signals of luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system without and with the introduction of NaBr or NaCl were investigated.As shown in Figure S11, the CL intensity decreased upon adding 1.0 mM Br − or Cl − .The addition of Br − or Cl − would squeeze out the adsorbed hydroxyl anions, decreasing the pH around micelle surface and inhibiting the CL reaction.Such a result was also observed in CTAB-modified montmorillonite improved CL system, the removal of halide counterions of CTAB layer had a positive effect on the CL amplification. [56]t should be noticed that the acidity of H 2 O 2 is very weak (pK a ∼ 11.7), that means the molar fraction of peroxide anion form increases with an increase in pH.However, under pH 7.4, the molar fraction of peroxide anion form was calculated to be 5.0 × 10 -5 , suggesting most H 2 O 2 exists as neutral molecule form.This is why luminol-Co 2+ -H 2 O 2 system only produces ultraweak CL emission.After the addition of C 8 -TPE-C 4 TAB, the adsorbed peroxide anions increased the local pH, which benefits the following CL reaction, and the consumed peroxide anions would be supplied by aqueous H 2 O 2 .The pH-dependent molar fractions of peroxide species were calculated and listed in Table S1.In a ward, the nonuniform pH distribution may impel the diffusion of H 2 O 2 from solution to micelle surface.However, the reaction rate was still slow due to the low molar fraction of peroxide anions.Interestingly, this process endows slow CL kinetics and long-lasting CL emission characters of luminol-Co 2+ -H 2 O 2 system.On the other hand, the addition of surfactant also increased the viscosity of solution.The increased viscosity inhibited the diffusion of peroxide anions and limited the following CL reaction.To verify the viscosity-mediated CL performances, the CL signals of luminol-Co 2+ -H 2 O 2 system were investigated in the absence and presence of glycerol.The CL kinetics became slow as the introduction of glycerol, as shown in Figure S12.These results demonstrated that C 8 -TPE-C 4 TAB-induced long-lasting CL emission might be attributed to the increased viscosity and the decreased fusion of reactants.As referred in Figure 3B, the addition of CTAB also caused increase of CL kinetics.The introduction of 2.0 mM CTAB would greatly increase the viscosity of solution, inhibiting the diffusion of reactants and limiting the following CL reaction.In this case, the introduction of CTAB also increases the CL kinetics through the viscosity-mediated reactant diffusion.Taken together, C 8 -TPE-C 4 TAB-mediated intensive and long-lasting CL emission is due to the synergistic effect of micelle-improved enrichment, the diffusion control of reactants and high-efficient CRET.
It is generally accepted that luminol possesses stronger emission under alkaline condition.To understand whether C 8 -TPE-C 4 TAB mediated intensive and long-lasting emission can be achieved under alkaline condition, we thus tested the CL signals of luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system under pH 9.8.It was found that intensive CL intensity with fast kinetics appeared, and the addition of C 8 -TPE-C 4 TAB did not cause the visible CL increment (Figure S13A).In addition, the CL spectrum with emission maximum of 425 nm was observed (Figure S13B).This characteristic CL emission peak was attributed to the excited luminol radicals, indicating the nonoccurrence of CRET.The fast CL kinetics under alkaline condition was aggrement with the previous reports. [57,58]

Long-lasting performances of the proposed CL system
To perform the long-lasting emission more clearly, the CL images were taken in real time by camera.For acquiring effective images, the concentrations of luminol, Co 2+ , H 2 O 2 , and C 8 -TPE-C 4 TAB were 60 μM, 10 nM, 2.5 mM, and 80 μM, respectively.The image was acquired with an interval of 4 min.Since the reaction occurs quickly, the 0 min point was not obtained; we herein provided the 0.5 min image as the first picture.As shown in Figure 7, bright blue emission was easily observed after the mixing of all CL substrates.Interestingly, there was no visible light attenuation even after 16 min reaction (Figure S14), suggesting this luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system was stable.Despite of the gradually decreased emission with the increasing reaction time, the CL emission was still observed after 60 min reaction.These results further verified the feasibility of the enhancement of intensive and long-lasting luminol CL by adding C 8 -TPE-C 4 TAB.The bright and long-lasting CL emission makes the luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system possible to construct a facile cold light for practical applications.In addition, the C 8 -TPE-C 4 TAB-improved CL signal requires less surfactant due to its ultralow CMC, which can form AIE micelle at low concentrations.Therefore, the development of various CL sensing platforms with high sensitivity based on this system is possible.

CONCLUSION
In summary, the intensive luminol-Co 2+ -H 2 O 2 system with long-lasting CL emission has been explored by introducing the C 8 -TPE-C 4 TAB AIE surfactant.The micelle-improved enrichment and diffusion control of reactants and CRET were critical to amplify the CL signals.Based on our results, the CL kinetics is enhanced through viscosity-mediated reactant diffusion.In contrast, the micelle-improved enrichment and CRET mainly contribute the intensive CL intensity.The visible CL emissions can be observed even after 60 min reaction at pH 7.4.This work not only develops a simple and glowtype luminol CL system but also provides a general tactic to construct the novel surfactant-mediated CL system by introducing AIE surfactant with ultralow CMC, which reduces the usage of surfactant and environmental pollution.Moreover, our work also provides a deep learning of the relationship between CMC and surfactant-improved CL signals.Therefore, we believe this strategy might benefit the exploration of green and effective CL systems for lighting, optical sensing and photocatalysis applications, etc.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no competing financial interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available on request from the corresponding author.

F I G U R E 1
Schematic illustration of C 8 -TPE-C 4 TAB-mediated intensive and long-lasting chemiluminescence (CL) emission of the luminol-Co 2+ -H 2 O 2 system.

F I G U R E 2
Chemiluminescence (CL) signals of the luminol-Co 2+ -H 2 O 2 system in the absence (A) and the presence (B) of C 8 -TPE-C 4 TAB.F I G U R E 3 (A) HRTEM image of 80 μM C 8 -TPE-C 4 TAB.(B) Chemiluminescence (CL) signals of the luminol-Co 2+ -H 2 O 2 system upon adding cetyltrimethylammonium bromide (CTAB) and C 8 -TPE-C 4 TAB.(C) CL signals of the luminol-Co 2+ -H 2 O 2 system upon adding CTAB with the different concentrations.(D) CL signals of the luminol-Co 2+ -H 2 O 2 system upon adding conventional cationic surfactants.
At pH 7.4, the C 8 -TPE-C 4 TAB micelle was positively charged F I G U R E 4 Time-dependent electron paramagnetic resonance (EPR) spectra of the luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system.

F I G U R E 5
Chemiluminescence (CL) emission spectra of the luminol-Co 2+ -H 2 O 2 system in the presence of cetyltrimethylammonium bromide (CTAB) micelle (A) and C 8 -TPE-C 4 TAB micelle (B).Inset pictures are the corresponding photographs.F I G U R E 6 Schematic illustration of cetyltrimethylammonium bromide (CTAB) micelle (left) and C 8 -TPE-C 4 TAB micelle (right) amplified chemiluminescence (CL) of the luminol-Co 2+ -H 2 O 2 system.the intensive CL of luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system.The schematic illustration of C 8 -TPE-C 4 TAB micelle-amplified CL was displayed in Figure 6.

F I G U R E 7
Time-dependent image of the luminol-Co 2+ -H 2 O 2 -C 8 -TPE-C 4 TAB system solution with 4 min interval acquisition.
This work was supported by the National Natural Science Foundation of China (grant numbers: U22A20397, 21974008, and 22074005) and the Natural Science Foundation of Beijing Municipality (grant number: 2202038).