Rare earth complexes chemiluminescence catalyzed by gold nanoparticles for fast sensing of Tb3+ and Eu3+

Jiajia Yuan, Xiaoya Fan, Jiacheng Yang, Xinfeng Zhang

State Key Lab of Geohazard Prevention & Geoenvironment Protection, College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China

Keywords:Gold nanoparticles Rare earth ions Chemiluminescence Multiplex analysis Fast sensing

ABSTRACT Developing multiplex sensing technique is of great significance for fast sample analysis.However, the broad emissions of most chemiluminescence (CL) luminophores make the multiplex CL analysis be diffi-cult.In this work, a simple and sensitive CL analytical method has been developed for the simultaneous determination of Tb3+ and Eu3+ thanking to their narrow band emission.The technique was based on a mixed CL system of periodate (IO4-)-hydrogen peroxide (H2O2)-rare earth complexes, in which the reactive oxygen species (ROSs) especially singlet oxygen (1O2) can transfer its energy to the complex of Tb3+/Eu3+-ethylenediaminetetraacetic acid disodium salt (EDTA) and then produce the characteristic emissions of Tb3+ and Eu3+ without cross-interference.The further experiment found that the CL emissions of Tb3+ and Eu3+ could be catalyzed by the gold nanoparticles (AuNPs) via enhancing the yield of 1O2.The CL intensities of Tb3+ (at 490 nm) and Eu3+ (at 620 nm) increased linearly with concentration of Tb3+ and Eu3+.After the optimization of the CL sensing conditions, the limits of detection (LOD)were 5.0×10-9 mol/L and 8.0×10-7 mol/L for Tb3+ and Eu3+, respectively.Finally, the method has been applied for measuring the contents of Tb3+ and Eu3+ in leaching solution of mine sample and Tb3+/Eu3+-contained nanomaterials with satisfactory results.The present system provides a new CL technique for multiplex sensing with simplicity and high sensitivity.

Chemiluminescence (CL) accompanying with light emission by the chemical reaction, displays the merits of high sensitivity, wide linear range and low background signal.It has been widespreadly used in food analysis [1,2], environmental monitoring [3,4] and chem-/biosensing [5],etc.The common CL systems include luminol [6,7], peroxyoxalate [8-10], potassium permanganate [11,12],cerium [13,14], periodate [15,16],etc.However, most of these CL systems are based on the emission of molecular luminophore.The broad emission band of molecular luminophore makes multicomponent analysis be difficult.Hence, developing a new CL system that can achieve multiple components is still highly desired.

In recent years, rare earth elements (REEs) have attracted widespread interest owing to their unique properties.The photoluminescence properties of lanthanide ions (Ln3+) mostly come from the f→f transitions of its 4f orbitals, which are well shielded from the environment by the outer 5s2and 5p6shells.Importantly, the Ln3+ions are usually characterized by narrow band emission, large Stokes shifts and long luminescence lifetime [17-19].Thus, the narrow band emission of Ln3+ions makes it promising for multi-component analysis.In most cases, the Ln3+excitation was achieved by light source [20-22].The background of the light sources can usually deteriorate the sensing sensitivity.

Recently, chemoexitation has demonstrated to be promising for the excitation of fluorophore [23,24], quantum dots [25],etc., since CL excitation can avoid light background and improve the signalto-noise ratio during detection [2].In recent years, REE ions especially Tb3+and Eu3+were also successfully chemoexcited by reaction system such as oxalate-hydrogen peroxide (H2O2), SO32--Eu/CeO2, periodate (IO4-)-H2O2[26-29].Among them, IO4-was the mostly used as the oxidant since it is water-soluble and colourless which evades the emission absorption problems [30,31].Nevertheless, the simultaneous extation of Tb3+and Eu3+by the CL reaction system has not been studied.Additionaly, the relatively weak CL emission from Tb3+/Eu3+-based CL systems limits their applications in chemo/bio-sensing.

Hence, in this work, we demonstrate the simultaneous excitation of Tb3+and Eu3+by IO4--H2O2CL reaction, and the analytical procedure is shown in Fig.S1 (Supporting information).The reactive oxygen species (ROSs) especically singlet oxygen (1O2) that generated by IO4--H2O2system can transfer its energy to the complex of Tb3+/Eu3+-ethylenediaminetetraacetic acid disodium salt(EDTA), then producing the CL emissions of Tb3+and Eu3+.More importantly, the emissions can be further enhanced by the catalysis of gold nanoparticles (AuNPs).The resulted strong characteristic CL emissions of Tb3+at 490 nm and Eu3+at 620 nm allow sensitive and simultaneous detection of Tb3+and Eu3+.This AuNPscatalyzed rare earth complexes CL system provides a new CL technique for multiplex sensing with simplicity and high sensitivity.

As shown in Fig.1A, Tb3+cannot yield CL emission in the presence of NaIO4, H2O2or EDTA alone, and even in the CL reaction between NaIO4and H2O2; the characteristic CL emissions of Tb3+appear only in the presence of NaIO4, H2O2and EDTA.It indicates that EDTA plays an important role in the luminescence of Tb3+,which can be attributed to the antenna effect of ligands [32,33].

Luminescence is an important property of rare earth ions, so we further verify whether other rare earth ions could also produce CL in this investigated system.The investigated ions include La3+,Ce3+, Pr3+, Nd3+, Sm3+, Dy3+, Er3+, Eu3+and Gd3+.It can be seen in Fig.1B, only Tb3+, Eu3+and Dy3+produce obvious CL emissions with Tb3+being the strongest, Eu3+the second strongest and Dy3+the smallest.This may be attributed to the difference in energy transfer efficiency between EDTA and these investigated ions, because only Tb3+, Eu3+and Dy3+also produce characteristic emissions in the fluorescence experiments (Fig.S2 in Supporting information).Although the emission peaks of Tb3+and Dy3+overlap at 490 nm, the intensity of 1 mmol/L Tb3+is much higher than that of 5 mmol/L Dy3+.Since the high CL eimissions of Tb3+and Eu3+-EDTA complexes, so we use the CL system for multiplex detection of Tb3+and Eu3+.

The results in Fig.1A show that ligand is critically important for the CL process of Tb3+.Therefore, we studied the sensitisation role of a variety of ligands in Tb3+and Eu3+CL emissions, including EDTA, 2′-deoxyadenosine 5′-triphosphate trisodium salt (ATP),diethylenetriamine-pentaacetic acid (DTPA), sulfosalicylic acid dehydrate (SAD), l-cysteine hydrochloride monohydrate (Cys), 2,6-pyridinedicarboxylic acid (DPA) and enrofloxacin (ENX).As depicted in Fig.1C, the three ligands (EDTA, ATP and DTPA) can produce strong CL emissions from the NaIO4-H2O2-Tb3+/Eu3+systems under their optimal conditions, the CL curves of Tb3+and Eu3+with a variety of ligands were shown in Fig.S3 (Supporting information).And EDTA was selected for subsequent experiments by considering the stability of the complexes as well as the cost effectiveness.

In recent years, the enhancement of CL by the catalysis of Au NPs has been studied [34-36], hence we also intend to explore whether AuNPs can contribute to the CL emissions of Tb3+and Eu3+.The synthetic AuNPs were characterized by transmission electron microscopy (TEM), ultravioletvisible (UV-vis) spectrophotometer and X-ray Photoelectron Spectroscopy (XPS).The data in Fig.2A revealed that they are spherical particles with an average diameter of about 13 nm and the UV absorption peak at 520 nm was illustrated in Fig.S4 (Supporting information).The overall XPS survey shows the presence of strong C 1s, O 1s and Au 4f core levels of citrate capped AuNPs (Fig.S5 in Supporting information).In Fig.2B, we found that the presence of AuNPs in the NaIO4-H2O2-EDTA-Tb3+solution caused an obvious enhancement in the CL emission.And AuNPs also has a catalysis effect on Eu3+CL emission (Fig.S6 in Supporting information).

The reaction of IO4-with H2O2may involve the generation of ROSs, such as hydroxyl radicals (?OH) and superoxide anion(?O2-) [31,37,38] which are important for the CL.This was confirmed by free radical scavenging experiments (Fig.S7 in Supporting information) and room temperature electron paramagnetic resonance (EPR) analysis (Figs.2C-E), which indicated that three free radicals (?OH,?O2-and1O2) are indeed generated in the NaIO4-H2O2system.Furthermore, it found that the addition of AuNPs to the NaIO4-H2O2system resulted in significantly increasing signals for 2,2,6,6-tetramethylpiperidinooxy (TEMPO) adduct (Fig.2E), suggesting that AuNPs could promote the production of1O2.It is generally believed that oxidation of H2O2by IO4-generates excited singlet oxygen molecular pair [(1O2)2?], which then produces a CL emission [31,39].So we have further explored the effect of AuNPs on the production of1O2using 1,3-diphenylisobenzofuran (DPBF)as an indicator.From Fig.2F, it can be concluded that AuNPs indeed contribute to the production of1O2because UV absorption of DPBF is reduced more seriously in the presence of AuNPs.These results indicated the good catalytic activity of AuNPs for enhanced generation of1O2in the investigated CL system.

For the catalytic effect of AuNPs, it is generally believed that the O-O bond of H2O2might be broken up into double?OH radicals and these radicals were further stabilized by AuNPsviapartial electron exchange interactions [40].And the generated?OH reacted with H2O2to facilitate the formation of HO2?, which immediately dissociated into?O2-at high pH values [41].Additionally,?OH and?O2-radicals could also be generated from the reaction of IO4-with dissolved O2.1O2could be produced from the reaction of H2O2or ROSs (e.g.?O2-or?OH) with IO4-[42].

It has been reported that1O2can transfer its energy to fluorescent substances by the intermolecular energy transfer process[43,44].However, lanthanide 4fN-4fNtransitions are Laporte forbidden, and the direct excitation of electrons in tripositive lanthanide ions is inefficient.Hence, a sensitizing chromophore is usually used as a ligand to chelate to the emissive lanthanide center [18,19].In such case, the rare earth ion can be excited by the antenna effect.So in this work, the fluorescence quantum yield of Tb3+is greatly enhanced through an intramolecular energy transfer when it forms complexes with EDTA (Fig.1A).

Fig.1.(A) Sensitization CL of Tb3+.(B) CL of various rare earth ions.(C) CL of different ligands in Tb3+ and Eu3+ system.Experimental conditions: NaIO4, 24 mmol/L; H2O2,18 mmol/L; EDTA, 35 mmol/L; all the ion are 5 mmol/L in (A and B), except for Tb3+, 1 mmol/L.In (C), NaIO4, 8-18 mmol/L; H2O2, 10.67-48 mmol/L; Tb3+, 1 μmol/L; ligands,0.2-13 mmol/L in Tb3+; NaIO4, 10-16 mmol/L; H2O2, 8-30 mmol/L; Eu3+, 20 μmol/L; ligands, 0.2-6.67 mmol/L in Eu3+.

Fig.2.(A) TEM of synthetic AuNPs.(B) The effect of AuNPs on the reaction of NaIO4-H2O2-EDTA-Tb3+.(C) DMPO/?OH adduct in NaIO4-H2O2 system in the absence or presence of AuNPs.(D) DMPO/?O2- adduct in NaIO4-H2O2 system in the absence or presence of AuNPs.(E) DMPO/1O2 adduct in NaIO4-H2O2 system in the absence or presence of AuNPs.(F) DPBF detection of 1O2 in presence of AuNPs.Experimental conditions: TbCl3, 1 mmol/L; EDTA, 35 mmol/L; H2O2, 18 mmol/L; NaIO4, 24 mmol/L;AuNPs, 2 nmol/L and DPBF, 200 μmol/L.

Fig.3.Possible chemiluminescence reaction mechanism.(A) Reaction bwtween NaIO4 and H2O2 to produce ROSs, (B) intermolecular energy transfer processes between 1O2 and rare earth complexes, and (C) Antenna effect between ligands and rare earth ions.

Hence, the AuNPs-catalzyed CL mechanism of the rare earth complex can be concluded as in Fig.3: (i) AuNPs catalyzed the production of1O2from NaIO4-H2O2system; (ii) the generated1O2transfered its energy to the ligand (i.e., EDTA) to reach the excited stateviaintermolecular energy transfer effect and (iii) the rare earth ion was excitedviathe antenna effect from ligand, being followed by5D4emission of Tb3+in the [Tb(EDTA)]-complex (Fig.1C).The catalytic activity of AuNPs was quite stable with a decline about 5.3% in a week.

We verified the feasibility of simultaneous detection of Tb3+and Eu3+by the constructed CL systems.The characteristic peaks of Tb3+and Eu3+are mainly at 490 nm and 540 nm, 595 nm and 610 nm (Fig.4A), respectively.And Dy3+in concentrations equal to Tb3+(0.5 mmol/L) produces nearly no luminescence.More importantly, these characteristic peaks (490 nm for Tb3+and 610 nm for Eu3+) do not mutually interfere in the mixed CL system of NaIO4-H2O2-Tb3+-Eu3+-EDTA.Hence, the CL system enables multiplex detection of Tb3+and Eu3+.

Fig.4.(A) Construction of a multi-component luminescent system.(B) The standard curves and detection limits of Tb3+.(C) The standard curves and detection limits of Eu3+.Experimental conditions: (A) Tb3+, 0.5 mmol/L; Dy3+, 0.5 mmol/L; Eu3+, 4 mmol/L; EDTA, 35 mmol/L; H2O2, 18 mmol/L and NaIO4, 24 mmol/L.(B) EDTA, 13 mmol/L;H2O2, 10.67 mmol/L; NaIO4, 13 mmol/L; AuNPs, 2 nmol/L and 480 nm filter.(C) EDTA, 13 mmol/L; H2O2, 10.67 mmol/L; NaIO4, 13 mmol/L; AuNPs, 2 nmol/L and 620 nm filter.

Some parameters that affect the CL emission were examined,including the concentrations of CL reagents and the pH of solutions (Figs.S8-S12 in Supporting information).Under optimal conditions, the linear response range of NaIO4-H2O2-EDTA-AuNPs for Tb3+and Eu3+were depicted in Figs.4B and C.It revealed that the CL intensity of NaIO4-H2O2-EDTA-AuNPs was gradually enhanced with the increase of the Tb3+concentration, and the linear response is ranging from 2.0×10-8mol/L to 1.0×10-6mol/L(R2=0.9941).And the CL intensity is also enhanced linearly with increasing Eu3+concentration in the range of 1.0×10-6mol/L to 5.0×10-5mol/L (R2=0.9967).The limits of detection (LOD) were calculated to be 5.0×10-9mol/L and 8.0×10-7mol/L for Tb3+and Eu3+respectively (3σ).Notably, the NaIO4-H2O2-EDTA-AuNPs CL system is much more sensitive than the corresponding fluorescence detection shown in Fig.S13 (Supporting information), in which the LODs for Tb3+and Eu3+were 6.7×10-6mol/L and 2.1×10-5mol/L, respectively.The results of comparing with other methods for the detection of Tb3+and Eu3+are summarized in Table S1 (Supporting information).These results showed that the NaIO4-H2O2-EDTA-AuNPs system had good linearity, high sensitivity and reproducibility.

To estimate the selectivity of the sensing system, we measured a variety of potentially interfering metal ions on the effect of the system, including Sm3+, Dy3+, Gd3+, Ce3+, Nd3+, La3+, Fe3+, Al3+,Zn2+, Ni2+, Co2+, Mg2+and Cu2+.Fig.S14 (Supporting information) demonstrated that only Tb3+and Eu3+can produce strong CL emission, negligible CL intensities were observed for other metal ions at the same concentrations.And the typical metal ions were further used to investigate the anti-interference ability of the system.As shown in Figs.5A and B, the CL signals for 0.5 μmol/L Tb3+and 20 μmol/L Eu3+were affected weakly by the examined ions.And for the determination of Tb3+, the tolerable concentration ratios for interference at the 10% level were over 1000 for K+,Na+and NH4+; 200 for La3+, 100 for Al3+and Gd3+, 50 for Ce3+and Eu3+, 10 for Mg2+, Fe3+, Ni2+, Cu2+and Sm3+, 5 for Co2+and Dy3+.With respect to Eu3+, the tolerable limit could reach more than 330 times for K+, Na+and NH4+, 20 for Mg2+, La3+, 13 for Al3+, 6 for Ni2+, Fe3+, Ce3+and Gd3+, 3 for Co2+, Cu2+and Dy3+.And the same concentration of Sm3+and Tb3+has no effect on CL of Eu3+.Hence, the CL system has good selectivity and antiinterference ability.

The applicability for real sample analysis was verified by determination of Tb3+and Eu3+in leaching solution of mine sample.Also, the Tb3+and Eu3+contents in Metal-organic frameworks(MOFs) materials were assessed by this sensing system.In Table 1, the measured values of Tb3+and Eu3+in the samples were in good agreement with those obtained by inductively coupled plasma mass spectrometry (ICP-MS).Hence, the system was practical for real sample analysis.

Fig.5.Anti interference experiment.(A) NaIO4-H2O2-EDTA-Tb3+.(B) NaIO4-H2O2-EDTA-Eu3+.Experimental conditions: EDTA, 13 mmol/L; H2O2, 10.67 mmol/L; NaIO4,13 mmol/L; AuNPs, 2 nmol/L; Tb3+, 0.5 μmol/L, interference ions, 0.5-33 μmol/L and 480 nm filter in (A).EDTA, 13 mmol/L; H2O2, 10.67 mmol/L; NaIO4, 13 mmol/L;AuNPs, 2 nmol/L; Eu3+, 20 μmol/L, interference ions, 20-3000 μmol/L, 620 nm filter in (B).

Table 1 Determination of Tb3+ and Eu3+ in samples.

In summary, rare earth complexes CL system that can catalyzed by AuNPs was developed for fast sensing of Tb3+and Eu3+.The1O2generated from NaIO4-H2O2system can transfer its energy to the complex of Tb3+/Eu3+-EDTA complex, following by the characteristic emissions of Tb3+and Eu3+.The AuNPs can act as a catalyst for boosting the generation of1O2in NaIO4-H2O2system.Thanking to their narrow band emission of rare earth ions, simultaneous determination of Tb3+and Eu3+can be easily achieved at 490 nm and 620 nm, respectively.This sensing system with the merits of high sensitivity, selectivity as well as simplicity for the determination of Tb3+and Eu3+, provides a new avenue for multicomponent CL analysis.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgments

The authors gratefully acknowledge the financial support from the Sichuan Science and Technology Project (No.2022NSFSC1087)and the Project of State Key Laboratory of Supramolecular Structure and Materials (No.sklssm2022034).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108155.