Terbium(III) As a Fluorescent Probe for Molecular Detection of Ascorbic Acid

chemosensors

Communication Terbium(III) as a Fluorescent Probe for Molecular Detection of Ascorbic Acid

Natalia Selivanova * and Yuriy Galyametdinov

Department of Physical and Colloid Chemistry, Kazan National Research Technological University, Kazan, Tatarstan 420111, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-843-231-4177

Abstract: Fluorescence analysis is a simple and a highly sensitive method for detection of small amounts of biologically active substances. In this study, a complexation of terbium(III) chelates with 1,10-phenanthroline and ascorbic acid (AA) and luminescent properties of complexes were investigated. The influence of pH and solubilization of complexes by micellar solutions of nonionic, cationic, and anionic surfactants on fluorescence was studied. The quenching effect of terbium ion fluorescence was detected upon an introduction of ascorbic acid. The quenching effect of the complex with mixed ligands Tb(1,10-phenanthroline)-AA allows for the detection of ascorbic acid with the limit of 7.4 × 10−5 mol·L−1.

Keywords: ﬂuorescence; terbium complex; ascorbic acid; quenching effect; micelles

1. Introduction Citation: Selivanova, N.; The ﬂuorescence analysis is a method that utilizes sensitized luminescence of lan- Galyametdinov, Y. Terbium(III) as a thanide complexes and is a highly sensitive technique broadly used in analytical chemistry Fluorescent Probe for Molecular and scientiﬁc research in biology and biomedicine [1,2]. This method is suitable for the Detection of Ascorbic Acid. detection of compounds that are capable of forming complexes with lanthanide ions Ln(III). Chemosensors 2021, 9, 134. In such complexes, an analyte acts as either a sensitizer or a quencher. Due to the unique https://doi.org/10.3390/ photophysical properties of Ln3+ complexes, such as a narrow emission band and a high chemosensors9060134 quantum yield of luminescence, the respective optical probes are widely used as highly sensitive analytical sensors for visualizing various lesions in cells and tissues, drug de- Academic Editor: Mark Lowry livery monitoring, and clinical analysis [3,4]. Recent studies report the applications of lanthanide-doped nanoparticles as biosensors for COVID-19 detection [5]. Received: 28 April 2021 In the aspect of molecular recognition of various drugs, the development of simple, Accepted: 7 June 2021 Published: 9 June 2021 rapid, and highly sensitive methods for their detection is highly demanded. Optical probes based on lanthanide complexes are of considerable interest, providing the required

Publisher’s Note: MDPI stays neutral accuracy, sensitivity, and rapidity of determination. Using sensitized luminescence of with regard to jurisdictional claims in lanthanides, drugs of various classes are determined, the molecules of which are capable 3+ published maps and institutional affil- of complexation with Ln ions: derivatives of aminobenzoic acid [6]; non-steroidal anti- iations. inflammatory drugs [7,8]; antibiotics of the tetracycline series [9]. Ascorbic acid, also known as vitamin C, performs the biological functions of a reducing agent and a coenzyme of metabolic processes. It is one of the main substances in the human diet which is necessary for the normal functioning of connective bone tissue. It plays an important role in such biological processes as the capture of free radicals, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. synthesis of collagen protein and a number of hormones including anti-stress, helps This article is an open access article maintain hemostasis, improves the body’s ability to absorb calcium, removes toxins, and distributed under the terms and regulates metabolism [10]. Various analytical methods are used for the detection of ascorbic conditions of the Creative Commons acid, including spectrophotometric, chromatographic, calorimetric, and electrochemical Attribution (CC BY) license (https:// ones [10–13]. Despite the fact that the listed methods are sensitive and very specific, there creativecommons.org/licenses/by/ are still some drawbacks. These methods are intricate and time-consuming, and some of 4.0/). them usually required specialized and expensive instruments. Compared to other detection

Chemosensors 2021, 9, 134. https://doi.org/10.3390/chemosensors9060134 https://www.mdpi.com/journal/chemosensors Chemosensors 2021, 9, 134 2 of 10

methods, fluorescent methods get more attention due to their good reproducibility and high sensitivity. Recently, the fluorescent method has been developed for AA sensing based on nitrogen-doped carbon dots [14]. Terbium(III) as a commonly used lanthanide ion in analytical chemistry. It is characterized by a large radius, versatile coordination geometry, and a high affinity to ligands containing oxygen or hybrid oxygen-nitrogen atoms. As summarized in the review [15,16], the Tb3+ sensitized fluorescence method is broadly used for the determination of pharmaceuticals. The detection of ascorbic acid based on the sensitized luminescence of terbium(III) was reported in [17,18]. The authors used the complex of terbium(III) nitrate with hydroxymethyl in methanol and the complex of terbium(III) chloride with acetylacetone in aqueous medium. Detection of the limits of ascorbic acid in the first case were 1.2 × 10−7 mol·L−1, in the second 8 × 10−3 mol·L−1. The aim of this work was to study the complexation between ascorbic acid and Tb3+ ions in the presence of the second ligand 1,10-phenanthroline and the micelles of different surfactants. The optimal terbium fluorescence conditions for ascorbic acid detection were determined.

2. Materials and Methods 2.1. Materials

Terbium nitrate pentahydrate Tb(NO3)35H2O, 1,10-phenanthroline C12H8N2(Phen), decaethylene glycol monododecyl ether C12H25O(CH2CH2O)10H(C12EO10), sodium dode- cyl sulfate CH3(CH2)10CH2OSO3Na(SDS), hexadecyltrimethylammonium bromide CH3(CH2)15NBrCH3 (HTAB) are commercial products from Aldrich and are used without additional processing. Ascorbic acid C6H8O6(AA) from Tatfarm Company. Solutions of all basic and auxiliary chemical reagents were prepared using double-distilled water. Acetate-ammonia buffer solution was prepared from 2 M CH3COOH and NH3 solutions.

2.2. Preparation of Samples Synthesis of the Tb(Phen) complex is carried out in an aqueous medium containing 1.6 × 10–3 M of surfactant based on a 1:1 molar ratio of the components. The sample of Phen was initially dissolved in 0.1 mL of ethanol. The solution was further stirred at room temperature for 2 h. The concentration of ascorbic acid in stock standard aqueous solutions is 4 × 10−3 M. To plot a calibration graph, 1 mL of a buffer solution with pH = 7.0 was added to the test tube. The following components were then added: 0.5 mL of Tb3 + 1 × 10−3 M salt −3 −3 solution, 0.5 mL of Phen 1×10 M, 0.5 mL of C12EO10 1.6 × 10 M. Various additives of AA are dissolved in 5 mL of the buffer solution to provide the concentration range of 1 × 10−6 M–1 × 10−4 M.

2.3. Detection of Ascorbic Acid in Tablets Five tablets containing 100 mg of ascorbic acid were ground into a fine powder. The powder was then divided into five equal parts. These parts were dissolved in water and filtered through a quantitative filter paper. The filtrates were collected and diluted with water to prepare solutions with different concentrations of ascorbic acid.

2.4. UV/Vis Electronic absorption spectra were recorded on a Lambda 35 Uv-Vis Spectrometer (PerkinElmer Ltd., Buckinghamshire, UK). We used quartz cells with an optical path length of 1 cm.

2.5. Fluorescence Fluorescence spectra were recorded on the Cary Eclipse (Varian Australia Pty Ltd., Mulgrave, Australia) spectroﬂuorimeter. The measurements were carried out in a quartz cells with a thickness of 1 cm; the signal was recorded at the angle of 90◦ to the excitation Chemosensors 2021, 9, 134 3 of 10

light. The ﬁlters were used in the automatic mode. The parameters of the excitation and emission slits were 20 nm and 5 nm, respectively. The emission spectra were obtained at the excitation wavelength of 303 nm. The excitation spectra were recorded by detecting the emission intensity of Tb3+ ions at 544 nm. For the emission lifetime, the ﬂuorescence intensities at λmax = 544 nm were recorded at different delay times. The experimental curves were exponentially approximated by Origin software.

2.6. DLS To measure the sizes of micellar aggregates, the method of dynamic light scattering was used. The studies were carried out on a Zetasizer Nano ZS apparatus with a helium– neon laser (633 nm, 4 mW) from (Malvern Instruments Ltd., Worcestershire, UK). Before measurement, the solutions were ﬁltered through a Millipore hydrophilic ﬁlter in the Millex HV Filter Unit with 0.45 µm pores. The light scattering angle was 173◦. The experiment was carried out at the temperature of 25 ◦C. The data on the sizes of micelles were taken from the Zetasizer Nano ZS software report.

2.7. pH The pH values of the solutions were measured by a Knick pH meter. The pH was varied using 1.0 M HCl and 0.1 M NaOH solutions.

3. Results and Discussion 3.1. Complexation of Tb(III) Ions with 1,10-Phenanthroline and Ascorbic Acid The efficiency of energy transfer and the intensity of sensitized luminescence depends on a number of factors, such as the nature of the ligand, the type of the lanthanide ion, solvent, pH of the medium, and the difference in energies between the triplet level of the ligand and the emitting level of the lanthanide. The fluorescence of Tb(III) ions occurs only from certain resonance levels. The energy of the triplet level of Phen is 22,075 cm–1; accordingly, this value is higher than the energy of the resonance level of the terbium ion at 20,950 cm–1 [15]. This makes possible the transfer of excitation energy from the organic ligand molecule to the terbium ion, which contributes to sensitized fluorescence. Spectrophotometric studies were carried out to assess the interactions of the Tb(Phen) complex with ascorbic acid. As can be seen from Figure1, the hypsochromic shift from 267 nm to 264 nm is Chemosensors 2021, 9, x FOR PEER REVIEW 4 of 11 observed in the presence of ascorbic acid. These data indicate a possible complexation process via interactions between lone oxygen pairs in ascorbic acid molecules and vacant terbium ion orbitals.

FigureFigure 1. 1. AbsorptionAbsorption spectra spectra of of complexes complexes 1.1 Tb(Phen). Tb(Phen) and and 2.2 Tb(Phen)-AA.. Tb(Phen)-AA.

FigureFigure 2 demonstrates the excitation an andd emission spectra of the Tb(Phen) and Tb(Phen)-AATb(Phen)-AA complexes. complexes. The The excitation excitation at at the the 303 303 nm nm wavelength wavelength leads leads to to the the formation formation of typical Tb3+ emission peaks at 489, 544, 585, and 620 nm. These peaks correspond to the of typical Tb3+ emission peaks at 489, 544, 585, and 620 nm. These peaks correspond to the following radioactive transitions of the Tb ions: 1. 5D4–7F6, 2. 5D4–7F5, 3. 5D4–7F4, and 4. 5D4– 7F3, respectively, and thus indicate an intramolecular energy transfer from Phen to Tb3+. Fluorescence at the wavelength λmax = 544 nm, which corresponds to the 5D4–7F5 transition, results in a green emission from the samples.

(a) (b)

Figure 2. (a) Excitation spectra; (b) Fluorescence spectra of systems: 1. Tb(Phen), 2. Tb(Phen)-AA, СAA = 1.4 × 10−4 mol·L−1.

In comparison with the Tb(Phen) system, the fluorescence spectra of the Tb(Phen)- AA system shows a significant decrease in intensity. The effect of fluorescence quenching is observed for the Tb (III) ions. A three-fold decrease in fluorescence intensity is detected if the concentration of the added ascorbic acid is C = 1.4 × 10−4 mol·L−1.

3.2. The Effect of Surfactants on the Fluorescence of Tb(III) Ions A well-known approach to increase the intensity of an analytical signal is the use of micelles, microemulsions, or nanoparticles in the fluorescence analysis [15]. As reported in [9], solubilization of lanthanide complexes in nanosized surfactant micelles is accompanied by an additional increase in the sensitized fluorescence intensity. This effect can be explained by an accumulation of complexes inside micelles, a convergence of reacting components, stabilization of complexes, and shielding of the luminescent particles from extraneous quenchers. On the other hand, the Tb(Phen) complex is poorly soluble in

Chemosensors 2021, 9, x FOR PEER REVIEW 4 of 11

Figure 1. Absorption spectra of complexes 1. Tb(Phen) and 2. Tb(Phen)-AA.

Chemosensors 2021, 9, 134 Figure 2 demonstrates the excitation and emission spectra of the Tb(Phen)4 ofand 10 Tb(Phen)-AA complexes. The excitation at the 303 nm wavelength leads to the formation of typical Tb3+ emission peaks at 489, 544, 585, and 620 nm. These peaks correspond to the 5 7 5 7 5 7 5 following radioactive transitions of the Tb ions: 1. D5 4– F76, 2. D54– F5,7 3. D4–5 F4, and7 4. D4– following radioactive transitions of the Tb ions: 1. D4– F6, 2. D4– F5, 3. D4– F4, and 4. 57F3, respectively,7 and thus indicate an intramolecular energy transfer from Phen to Tb3+. D4– F3, respectively, and thus indicate an intramolecular energy transfer from Phen to Fluorescence3+ at the wavelength λmax = 544 nm, which corresponds to the 5D4–7F5 transition,5 7 Tb . Fluorescence at the wavelength λmax = 544 nm, which corresponds to the D4– F5 transition,results in a results green emission in a green from emission the samples. from the samples.

(a) (b)

−4−4 −−11 FigureFigure 2.2. ((aa)) ExcitationExcitation spectra; spectra; ( b(b)) Fluorescence Fluorescence spectra spectra of of systems: systems:1. Tb(Phen),1. Tb(Phen),2. Tb(Phen)-AA,2. Tb(Phen)-AA, CAA СAA=1.4 = 1.4× 10× 10 mol mol·L·L .

In comparisoncomparison withwith the the Tb(Phen) Tb(Phen) system, system, the the fluorescence fluorescence spectra spectra of theof the Tb(Phen)-AA Tb(Phen)- systemAA system shows shows a significant a significant decrease decrease in intensity. in intensity. The The effect effect of fluorescenceof fluorescence quenching quenching is observedis observed for for the the Tb Tb (III) (III) ions. ions. A A three-fold three-fold decrease decrease in in fluorescence fluorescence intensity intensity is detectedis detected if −4 −1 theif the concentration concentration of of the the added added ascorbic ascorbic acid acid is Cis =C 1.4= 1.4× ×10 10−4 molmol·L·L−1. .

3.2. The Effect of Surfactants on the Fluorescence of Tb(III) Ions A well-knownwell-known approachapproach to to increase increase the the intensity intensity of of an an analytical analytical signal signal is the is the use use of mi- of celles,micelles, microemulsions, microemulsions, or nanoparticles or nanoparticles in the in ﬂuorescence the fluorescence analysis analysis [15]. As[15]. reported As reported in [9], solubilizationin [9], solubilization of lanthanide of lanthanide complexes complexes in nanosized in nanosized surfactant surfactant micelles ismicelles accompanied is accom- by an additional increase in the sensitized ﬂuorescence intensity. This effect can be explained panied by an additional increase in the sensitized fluorescence intensity. This effect can by an accumulation of complexes inside micelles, a convergence of reacting components, be explained by an accumulation of complexes inside micelles, a convergence of reacting stabilization of complexes, and shielding of the luminescent particles from extraneous components, stabilization of complexes, and shielding of the luminescent particles from quenchers. On the other hand, the Tb(Phen) complex is poorly soluble in water. Micellar extraneous quenchers. On the other hand, the Tb(Phen) complex is poorly soluble in systems are, therefore, expected to enhance the solubility of complexes and increase the stability or their analytical signal. These effects depend on the nature of surfactants. In this regard, we studied the effect of nonionic, anionic, and cationic surfactants on the properties of complexes. The characteristics of surfactants used in this work are summarized in Table1. The concentrations of surfactants in solutions were higher than the critical micelle concentrations (CMC).

Table 1. Surfactant characteristics.

Working Concentration −1 Tb(Phen)-AA Complex Surfactant −1 CMC, mol·L d, nm Csurf, mol·L Diameter, nm −3 −5 C12EO10 1.6 × 10 9.1 × 10 [19] 3 ± 0.92 7.5 ± 1.60 SDS 8.3 × 10−2 8.1 × 10−3 [20] 5 ± 1.07 5.5 ± 1.12 HTAB 3.0 × 10−2 9.1 × 10−4 [20] 6 ± 1.27 1.7 ± 0.32

The effect of the Tb(Phen)-AA complex additives on the size of micellar aggregates depends on the type of surfactant (Figure3). Introduction of the Tb(Phen)-AA complex into HTAB micellar solutions reduces the size of micelles from 6 nm to 1.7 nm, indicating that no solubilization of added complexes occur in this system. The sizes of SDS and C12EO10 ChemosensorsChemosensorsChemosensorsChemosensorsChemosensors 20212021 20212021 2021,, 99,,, , 9 9xx,, FORx9xFOR , 2021FOR FORx FOR PEERPEER, PEERPEER9, PEERx FOR REVIEWREVIEW REVIEWREVIEW REVIEW PEER REVIEW 55 55ofof of 5of 11 11 of1111 11 5 of 11

water.water.water.water. MicellarMicellar Micellarwater. Micellar Micellarsystemssystems systems systems are,systemsare, are, are, therefore,therefore, therefore, therefore, are, therefore, expeexpe expe expectedctedctedcted expe toto to enhanceenhanceto ctedenhance enhance to enhancethethe the the solubilitysolubility solubility solubility the solubility ofof of complexescomplexesof complexes complexes of complexes andandandand increaseincrease increase increaseand theincreasethe the the stabilitystability stability stability the orstabilityor or their theiror their their analytical analyticalor analytical analyticaltheir analytical signal.signal. signal.signal. signal. TheseThese TheseThesesignal. These effectseffects effectseffects These effects dependdepend dependdependeffects depend on ondepend onon theonthe thethe the naturenature naturenatureon nature the ofof ofofnature of of surfactants.surfactants.surfactants.surfactants.surfactants.surfactants. InIn InIn this thisIn thisthis this regard,regard, regard,regard,In regard, this wewe regard,wewe westudiedstudied studiedstudied studied we the thestudied thethe the effecteffect effecteffect effect the ofof ofof nonionic, effectnonionic,of nonionic,nonionic, nonionic, of nonionic, anionic,anionic, anionic,anionic, anionic, andandanionic, andand and cationiccationic cationiccationic cationic and surfac-surfac-cationic surfac-surfac- surfac- surfac- tantstantstantstantstants onon onon theonthetants thethe the propertiesproperties properties propertieson properties the properties ofof ofof complexes. complexes.of complexes.complexes. complexes. of complexes. TheTheTheThe characteristicscharacteristics characteristics characteristicsThe characteristics ofof of surfactants surfactantsof surfactants surfactants of surfactants usedused used used inin in this thisinused thisthis this workwork workworkin work this areare areare work are summarizedsummarized summarizedsummarized summarized are summarized inin inin Table Tablein TableTable Table 1.in1. 1.1. TheTheTable 1.TheThe The 1. The concentrationsconcentrationsconcentrationsconcentrationsconcentrations ofof of surfactants surfactantsof surfactants surfactants of surfactants inin in solutions solutionsin solutions solutions in solutions werewere were were higherhigher higher werehigher thanthan thanhigher than thethe the the criticalthancritical critical critical the micellemicelle micellecritical micelle concentra-concentra- micelleconcentra- concentra- concentra- tionstionstionstionstions (CMC).(CMC). (CMC).(CMC). (CMC).tions (CMC).

TableTableTableTable 1.1. 1. SurfactantSurfactant Table 1.Surfactant Surfactant 1. Surfactant characteristics.characteristics. characteristics. characteristics. characteristics.

WorkingWorkingWorkingWorking ConcentrationWorkingConcentration Concentration Concentration Concentration −1−1 −1 Tb(Phen)-AATb(Phen)-AATb(Phen)-AATb(Phen)-AATb(Phen)-AA ComplexComplex Complex Complex DiameteComplexDiamete Diamete Diameterr ,Diamete, r r,, r, r, SurfactantSurfactantSurfactantSurfactant CMC,CMC,CMC, mol·Lmol·L mol·LCMC,−1 − −11 −d,mol·Ld,1 d, nmnm nm d, nm SurfactantSurfactant −−11−−11 −CMC,1 CMC, mol·L mol·L d,d, d,nmnm nm ССsurfСsurfsurfsurfС,, mol·Lsurfmol·L,, mol·Lmol·L, mol·LСsurf −1, −mol·L1 nmnmnm nm nm −−33−−33 −3 −−55−−55 −5 CC12C12EO12EO12CEO121010EO 10 10 C10 12EO10 1.61.61.6 ××1.6 ×1010 10× 10 −31.6 −3 × 10 9.1 9.1 9.1 9.1 ×× 9.1 ×10×10 1010× 10 −[19][19]5 9.1 [19][19]−5 [19]× 10 3 3 3± 3± ±0.92±0.92 3[19] 0.920.92± 0.92 3 ± 0.92 7.5 7.5 7.5 7.5 ±± 7.5 ±1.60±1.60 1.601.60± 1.60 7.5 ± 1.60 −2 −3 SDSSDSSDSSDS SDS 8.38.38.3 ××8.3 ×1010 10×−−2 2 10− −228.3 −2 × 10 8.1 8.1 8.1 8.1 ×× 8.1 ×10×10 1010×−−3 3 10− −[20][20]33 8.1 [20][20]−3 [20]× 10 5 5 5± 5± ±1.07±1.07 5[20] 1.071.07± 1.07 5 ± 1.07 5.5 5.5 5.5 5.5 ±± 5.5 ±1.12±1.12 1.121.12± 1.12 5.5 ± 1.12 −2 −4 HTABHTABHTABHTAB HTAB 3.03.03.0 ××3.0 ×1010 10×−−2 2 10− −223.0 −2 × 10 9.1 9.1 9.1 9.1 ×× 9.1 ×10×10 1010×−−4 4 10− −[20][20]44 9.1 [20][20]−4 [20]× 10 6 6 6± 6± ±1.27±1.27 6[20] 1.271.27± 1.27 6 ± 1.27 1.7 1.7 1.7 1.7 ±± 1.7 ±0.32±0.32 0.320.32± 0.32 1.7 ± 0.32

Chemosensors 2021, 9, 134 TheTheTheThe effecteffect effect effectThe ofof of the effecttheof the the Tb(Phen)-AATb(Phen)-AA Tb(Phen)-AA of Tb(Phen)-AA the Tb(Phen)-AA complexcomplex complex complex complexadditiadditi additi additivesvesves additi ves onon on theonthe theves the sizesize sizeon size ofofthe of micellar micellarof micellarsize micellar of aggregatesmicellaraggregates aggregates aggregates5 of aggregates 10 dependsdependsdependsdepends dependsonon on theonthe the the typetype typeon type theofof of surfactant surfactantof typesurfactant surfactant of surfactant (Figure(Figure (Figure (Figure 3). 3). (Figure3). Introduction3).Introduction Introduction Introduction 3). Introduction ofof of thetheof the the Tb(Phen)-AATb(Phen)-AA Tb(Phen)-AA of Tb(Phen)-AA the Tb(Phen)-AA complexcomplex complex complex complex intointointointointo HTABHTAB HTABHTAB HTABinto micellarmicellar micellarmicellarHTAB micellar solutionsmicellarsolutions solutionssolutions solutions solutions reducesreduces reducesreduces reduces the thereduces thethe the sizesize sizesize size of ofthe of micelles micellesof micellessize micelles of frommicellesfrom from from 66 6nmnm nm6from nm toto to 1.7 1.76to 1.7 nm nm,1.7nm, nm, tonm, indicatingindicating 1.7indicating indicating nm, indicating thatthatthatthatthat nono nono nosolubilizationsolubilization that solubilizationsolubilization solubilization no solubilization ofof ofof addedaddedof addedadded added of complexescomplexes complexesaddedcomplexes complexes complexes occuroccur occuroccur occur inin in in occur thisthisin thisthis this system.system. insystem.system. system. this TheThesystem. TheThe The sizessizes sizessizes sizesThe ofof ofof SDSSDSsizesof SDSSDS SDS and and ofandand and SDS and aggregates in the presence of this complex are 5.5 nm and 7.5 nm, respectively. In the case CCC1212EO12EO12CEO1210EO10 10 10aggregatesaggregatesC aggregates10aggregates12 aggregatesEO10 aggregates inin inin thethe in thethe the presencepresence presencepresence in presence the presence ofof ofof thisthis of thisthis this complexcomplex complexcomplex of complex this complexareare areare are5.55.5 5.55.5 nm5.5nm nmnmare nm andand andand5.5 and 7.5 7.5nm 7.57.5 nm,7.5nm, andnm,nm, nm, respectively.respectively. 7.5respectively.respectively. respectively. nm, respectively. InIn InIn In In thetheofthethe anthe casecase casecase anionic case oftheof ofof an an of caseanan surfactant anionicananionic anionicanionic ofanionic an surfactant surfactant anionic surfactantsurfactant (SDS), surfactant thesurfactant (SDS),(SDS), (SDS),(SDS), size (SDS), of thethe thethe micelles(SDS), the sizesize sizesize size ofofthe ofof is micelles micellesof micelles almostmicellessize micelles of is insensitivemicellesis is is almostalmost isalmostalmost almost is insensitiveinsensitive insensitivealmost toinsensitive insensitive additions insensitive toto toto addi- ofaddi-to addi-addi-the addi- to addi- Tb(Phen)-AA complex. The micelles of a nonionic surfactant (C12EO10), however, grow tionstionstionstionstions ofof ofof the thetionsof thethe the Tb(Phen)-AATb(Phen)-AA Tb(Phen)-AATb(Phen)-AA of Tb(Phen)-AA the Tb(Phen)-AA complex.complex. complex.complex. complex. complex. TheThe TheThe The micellesmicelles micelles micelles The ofmicellesof of a aof anonionicnonionic nonionica nonionic of a nonionic surfactantsurfactant surfactant surfactant surfactant (C(C (C1212 (CEO12EO12EO1210EO10), 10),10(C ), ),how-10how- 12),how-how-EO how-10), how- from 3 nm to 7.5 nm. An increase in the size of C12EO10 micelles may indicate that they ever,ever,ever,ever, growgrow grow growever, fromfrom from growfrom 33 3nmnm nmfrom3 nm toto to 7.5 7.53to 7.5 nm 7.5nm.nm. nm. tonm. An An 7.5An Anincrease increasenm. increase increase An inincreasein in the thein the the sizesize size in size of ofthe of C Cof C12size12 EO12EO12CEO12 10ofEO10 10 10micelles micellesC 10micellesmicelles12 micellesEO10 maymicellesmay maymay may indicateindicate indicateindicate indicatemay indicate solubilize the Tb(Phen)-AA complex. thatthatthatthatthat theythey theythey they that solubilizesolubilize solubilizesolubilize solubilizethey solubilize thethe thethe the Tb(Phen)-AATb(Phen)-AA Tb(Phen)-AATb(Phen)-AA Tb(Phen)-AA the Tb(Phen)-AA complex.complex. complex.complex. complex. complex.

FigureFigureFigureFigureFigure 3.3. 3. ParticleFigureParticle 3.Particle Particle 3. sizesize sizeParticle size distributiondistribution distribution distribution size distribution curves:curves: curves: curves: HTABHTAB HTABcurves: HTAB ( ( ( );HTAB );( );SDSSDS SDS ); SDS ( ( ( ); );( );C C12EO10SDSC 12C );12EO 12EO12C EO(121010EO 10 );10(( (C10( ( );Tb(Phen)-AA-HTAB 12 );Tb(Phen)-AA-HTAB( );Tb(Phen)-AA-HTABEO );Tb(Phen)-AA-HTAB );Tb(Phen)-AA-HTAB10 ( );Tb(Phen)-AA-HTAB (( (( ( ( ( );Tb(Phen)-AA-SDS);Tb(Phen)-AA-SDS);Tb(Phen)-AA-SDS);Tb(Phen)-AA-SDS);Tb(Phen)-AA-SDS);Tb(Phen)-AA-SDS (( ( ( ); );( );); Tb(Phen)-AA-CTb(Phen)-AA-C Tb(Phen)-AA-C Tb(Phen)-AA-C12EO10Tb(Phen)-AA-C); Tb(Phen)-AA-C ( ); Tb(Phen)-AA-C1212EO12EO12EO121010EO 10(). 10(). ().().10 12 ((). EO ). 10 ().

Chemosensors 2021, 9, x FOR PEER REVIEWTo ToToTo characterizecharacterize characterizeTo characterize thethe the effecteffect effect the ofof of surfactanteffect surfactant surfactant of surfactant mimi micelles micellescellescelles onmion on celles thethe the luminescent luminescent luminescenton the luminescent propertiesproperties properties properties6 of of ofof the the 11the of the ToTo characterize characterize the the effect effect of of surfactant surfactant mi micellescelles on on the the luminescent luminescent properties properties of of the the Tb(Phen)-AATb(Phen)-AATb(Phen)-AATb(Phen)-AATb(Phen)-AA complex,complex, complex, complex, complex, complex, itsits its its fluorescenceitsfluorescence fluorescence fluorescence fluorescence its fluorescence spectraspectra spectra spectra spectra (Figure (Figure (Figure spectra(Figure (Figure S1) S1) (FigureS1) S1)andand and and lifetimelifetime S1) lifetime lifetime and (Figure(Figure lifetime (Figure (Figure 44) 4) (Figure)4) were werewere 4)were were 4) were 5 55 55 77 77 5 55 555 7 77 77 7 5 7 analyzed.analyzed.analyzed.analyzed.analyzed.analyzed. TheThe The The ratio ratio ratio ratio The of of of the the of ratiothe the intensitiesintensities intensities of intensities the intensities of ofof of the the theof the transitions the transitionstransitions transitions of transitions the transitions I( I(I( DI(DD 45I(D4−4−5−4D4−FF475F5− )/I()/I(I()/I(755F)/I()/I(5D)/I(DD45D−44−−54FD4−F−F547)/I(6F6−)) 76F6 mayF))may6 D6maymay)) may4may− bebeF bebe6 ) beaa be may aacrite-crite- a crite-acrite- cri- crite- be a crite- rionrionterionrionrionrion Figureforfor forfor for forassessingassessingrion assessingassessing assessing assessing4, forhowever, assessing thethe thethe the the luminescenceluminescence luminescence luminescence luminescencereveals luminescence the luminescence the effectmonochromonochro monochromonochro monochromaticity monochro of sumonochromaticitymaticityrfactantsmaticitymaticity ofmaticityof on ofof the theof thethethe the terbiuterbiu terbium(III)terbiuluminescence of terbiu them(III)m(III)m(III) terbium(III) complexescomplexes complexescomplexesm(III) complexeslifetimes. complexes [19].[19]. [ [19].19 It [19].]. [19]. WeWeWe We thereforetherefore therefore thereforeWe therefore estimated estimatedestimated estimated estimated estimated the thethe the fluorescence the fluorescencefluorescence fluorescence fluorescence the fluorescence efficiency efficienefficien efficien efficiencycy of cyefficien theseofofcy of these theseof these complexescythese complexescomplexesof complexes thesecomplexes as complexes the asas as ratio thetheas the the ratioratio of ratio as theratio theofof mainof the theof ratiothe the of the should be noted that the luminescence5 deca7 y of5 the 7Tb(Phen)-AA complexes in micellar energy transitionmain energy intensities transitionη = intensities I( D −55 55F445 η)/I(477 7=755 I(75D5D55 45−5−4475F47F757)/I(766)76 at5D the4−7F wavelengths6) at the wavelengths of λ = 544 of nm λ = 544 nm solutionsmainmainmainmain energyenergy energy energyis approximated transitiontransition transition transition intensitiesintensities intensities byintensities a monoexponential ηη η == 4 =η=I(I( I(I(=DD I(D−5−4D−FF4F−)/I()/I(5F)/I()/I(5 )/I(4curveDDD−−4D−FF64 F−fitting.)) 6 atF)at) at6at ) thethe at thethe the wavelengthswavelengths wavelengthswavelengths wavelengths ofof ofof λλ of λ == =λ=544544 544544= 544 nmnm nmnm nm andandandandand λλ λ == =λ=489489 489and 489489= 489 nm.nm. nm. λnm.nm. =nm. 489 nm. TheTheTheThe fluorimetricfluorimetric fluorimetric fluorimetricThe fluorimetric datadata data data showshow show showdata thatthat that showthat thethe thethe the fluorescencethatfluorescence fluorescencefluorescence fluorescence the fluorescence efficiencyefficiency efficiencyefficiency efficiency efficiency ηη η ofof ηofof thetheof thethe the ηTb(Phen)-AATb(Phen)-AA Tb(Phen)-AATb(Phen)-AAof Tb(Phen)-AA the Tb(Phen)-AA complexescomplexescomplexescomplexescomplexes inin in thethe in the the micellarmicellar micellar in micellar the solutionsmicellarsolutions solutions solutions solutions ofof of surfacta surfactaof surfacta surfacta ofntsnts surfactants ntsdoesdoes does does notntsnot not not doeschangechange change change not significantlysignificantly changesignificantly significantly significantly (Table(Table (Table (Table 2).2). 2). (Table 2). 2).

FigureFigure 4. 4. FluorescenceFluorescence decay decay curves curves of of the the Tb(Phen)-AA Tb(Phen)-AA complexes complexes solubilized solubilized in in the the surfactant surfactant solutions: 1. C12EO10; 2. SDS; 3. HTAB. solutions: 1.C12EO10; 2. SDS; 3. HTAB.

TableThe fluorimetric2 represents dataluminescent show that characteri the fluorescencestics of the efficiency Tb(Phen)-AAη of thecomplex Tb(Phen)-AA solubil- izedcomplexes in surfactant in the solutions. micellar solutions of surfactants does not change significantly (Table2). Figure4, however, reveals the effect of surfactants on the luminescence lifetimes. It Tableshould 2. The be notedlifetime that and the the luminescence fluorescence efficiency decay of of the the Tb(Phen)-AATb(Phen)-AA complex complexes in microhetero- in micellar 5 7 geneoussolutions media is approximated (λ = 545 nm, transition by a monoexponential D4 → F5). curve fitting.

Surfactant Water C12EO10 SDS HTAB τ, µs 160 ± 16 194 ± 19 40 ± 4 187 ± 19 η 1.83 ± 0.09 1.98 ± 0.1 1.98 ± 0.1 1.91 ± 0.09

As we can see from Table 2, the presence of C12EO10 micelles increases the lifetime of complexes. These data are consistent with our earlier works, which characterized interactions between lanthanide ions with nonionic surfactants [21,22]. The lifetime of the Tb(Phen)-AA complex is longer in the С12ЕО10 solution. According to the concepts developed in our studies, these effects can be caused by interactions between lone electron pairs in oxygen atoms of surfactant oxyethylated groups with the vacant orbitals of the Tb3+ ions, so these ligands fill the coordination spheres of the lanthanide metal ions. A longer lifetime can be therefore associated with the solubilization of complexes in micellar solutions and a reduction of radiationless losses, so we used a nonionic surfactant C12EO10 for further studies.

3.3. The Effect of pH The intensity of sensitized fluorescence of the Ln(III) complexes depends significantly on the pH of medium. This effect is associated with the acid-base equilibria of a ligand and complex as well as the hydrolysis of lanthanide ions [8,23]. Considering this, we studied the influence of pH of medium on the fluorescence efficiency of the complexes. As can be seen from Figure 5, the fluorescence parameter η of the analyzed complexes first increases with the increase in pH, reaching a maximum, and then decreases. For the Tb (Phen) complex, a small plateau is observed in the pH range 5.4–7.2; for the Tb(Phen)- Аsc complex, the maximum value of the parameter η is observed at pH 7–8. An increase in pH leads to a sharp decrease in the fluorescence intensity. This is probably due to decomposition of Tb(Phen) complexes at pH > 7.2 and Tb(Phen)-AA) complexes at pH > 8 with the resulting formation of terbium hydroxide Tb(OH)3. A low intensity of fluorescence of these complexes under study in the acidic region indicates that such conditions

Chemosensors 2021, 9, 134 6 of 10

Table 2. The lifetime and the ﬂuorescence efﬁciency of the Tb(Phen)-AA complex in microheteroge- 5 7 neous media (λ = 545 nm, transition D4 → F5).

Surfactant Water C12EO10 SDS HTAB τ, µs 160 ± 16 194 ± 19 40 ± 4 187 ± 19 η 1.83 ± 0.09 1.98 ± 0.1 1.98 ± 0.1 1.91 ± 0.09

Table2 represents luminescent characteristics of the Tb(Phen)-AA complex solubilized in surfactant solutions. As we can see from Table2, the presence of C 12EO10 micelles increases the lifetime of complexes. These data are consistent with our earlier works, which characterized interactions between lanthanide ions with nonionic surfactants [21,22]. The lifetime of the Tb(Phen)-AA complex is longer in the C12ЕО10 solution. According to the concepts developed in our studies, these effects can be caused by interactions between lone electron pairs in oxygen atoms of surfactant oxyethylated groups with the vacant orbitals of the Tb3+ ions, so these ligands ﬁll the coordination spheres of the lanthanide metal ions. A longer lifetime can be therefore associated with the solubilization of complexes in micellar solutions and a reduction of radiationless losses, so we used a nonionic surfactant C12EO10 for further studies.

3.3. The Effect of pH The intensity of sensitized fluorescence of the Ln(III) complexes depends significantly on the pH of medium. This effect is associated with the acid-base equilibria of a ligand and complex as well as the hydrolysis of lanthanide ions [8,23]. Considering this, we studied the influence of pH of medium on the fluorescence efficiency of the complexes. As can be seen from Figure5, the fluorescence parameter η of the analyzed complexes first increases with the increase in pH, reaching a maximum, and then decreases. For the Tb (Phen) complex, a small plateau is observed in the pH range 5.4–7.2; for the Tb(Phen)-Asc Chemosensors 2021, 9, x FOR PEER REVIEWcomplex, the maximum value of the parameter η is observed at pH 7–8. An increase7 of 11 in pH leads to a sharp decrease in the fluorescence intensity. This is probably due to decomposition of Tb(Phen) complexes at pH > 7.2 and Tb(Phen)-AA) complexes at pH > 8 with the resulting formation of terbium hydroxide Tb(OH)3. A low intensity of fluorescence do not favor formation of complexes. The Tb(Phen)-AA complex maintains its fluorescent of these complexes under study in the acidic region indicates that such conditions do propertiesnot favor in formation a wider range of complexes. of pH = The2–10 Tb(Phen)-AAand demonstrates complex the maintainsmaximum itsfluorescence fluorescent intensityproperties at pH in a= wider7–8. To range eliminate of pH the = effect 2–10and of adding demonstrates AA on pH, the we maximum further studied fluorescence the quenchingintensity ateffect pH =of 7–8. Tb(Phen)-AA To eliminate complex the effect at pH of adding = 7 using AA acetate-ammonia on pH, we further buffer studied solu- the tion.quenching effect of Tb(Phen)-AA complex at pH = 7 using acetate-ammonia buffer solution.

FigureFigure 5. 5.TheThe dependence dependence of of the the luminescenc luminescencee efficiency efﬁciency on on the the pH pH in in C C1212EOEO1010 micelles:micelles: 1. 1Tb(Phen). Tb(Phen) −5 −5 andand 2. 2Tb(Phen)-AA,. Tb(Phen)-AA, CAA CAA = 1.2= 1.2× 10× 10M. M.

3.4. Quenching Effect Calibration Graph and Detection Limits of Ascorbic Acid To optimize the conditions for detecting ascorbic acid by a fluorometric method, to increase the detection sensitivity, and to lower the detection limit, the excitation and fluorescence spectra of Tb(Phen) complexes were studied in the presence of ascorbic acid in the concentration range of 1.6 ×·10−6–3.8·× 10−4 M. As we can see from the fluorescence spectra (Figure 6), the fluorescence intensity decreases upon the addition of acid.

Figure 6. Fluorescence spectra of the Tb(Phen) complex with various additives of ascorbic acid (рН = 7).

Figure 7 illustrates a Stern–Volmer plot for fluorescence quenching of Tb(Phen) by ascorbic acid. The fluorescence intensities were measured at λ = 544 nm.

Chemosensors 2021, 9, x FOR PEER REVIEW 7 of 11

do not favor formation of complexes. The Tb(Phen)-AA complex maintains its fluorescent properties in a wider range of pH = 2–10 and demonstrates the maximum fluorescence intensity at pH = 7–8. To eliminate the effect of adding AA on pH, we further studied the quenching effect of Tb(Phen)-AA complex at pH = 7 using acetate-ammonia buffer solution.

Chemosensors 2021, 9, 134 7 of 10 Figure 5. The dependence of the luminescence efficiency on the pH in C12EO10 micelles: 1. Tb(Phen) and 2. Tb(Phen)-AA, CAA = 1.2 × 10−5M.

3.4. Quenching Effect Calibration Graph and Detection Limits of Ascorbic AcidAcid To optimize the conditions conditions for for detecting detecting ascorbic ascorbic acid acid by by a fluorometric a fluorometric method, method, to toincrease increase the the detection detection sensitivity, sensitivity, and and to lower to lower the thedetection detection limit, limit, the theexcitation excitation and andflu- fluorescenceorescence spectra spectra of ofTb(Phen) Tb(Phen) complexes complexes were were studied studied in inthe the presence presence of ofascorbic ascorbic acid acid in −6 −4 inthe the concentration concentration range range of of1.6 1.6 ×·10×·−610–3.8·×–3.8 10·×−4 M.10 M. As we can can see see from from the the fluorescence fluorescence spectr spectraa (Figure (Figure 6),6 ),the the fluorescence fluorescence intensity intensity de- decreasescreases upon upon the the addition addition of ofacid. acid.

Figure 6. FluorescenceFluorescence spectra of the Tb(Phen) complex with various additives ofof ascorbicascorbic acidacid ((pHрН == 7). Chemosensors 2021, 9, x FOR PEER REVIEW 8 of 11 Figure7 7 illustrates illustrates aa Stern–VolmerStern–Volmer plot plot for for ﬂuorescence fluorescence quenching quenching ofof Tb(Phen) Tb(Phen) byby ascorbic acid. The ﬂuorescencefluorescence intensities were measuredmeasured atat λλ == 544544 nm.nm.

Figure 7. Stern–Volmer plotplot forfor Tb(Phen)Tb(Phen) quenchingquenching byby ascorbicascorbic acid.acid.

According to current current concepts concepts of of fluorescence ﬂuorescence quenching quenching types, types, a possible a possible process process in inthis this case case can can be bedynamic dynamic quenching quenching with with emissi emissionlessonless transfer transfer of electron of electron excitation excitation en- energyergy described described by bya Stern–Volmer a Stern–Volmer equation equation (Equation (Equation (1)) for (1)) a forbimolecular a bimolecular reaction reaction 𝐷∗ + ∗ ∗ 𝐴→𝐷+𝐴D + A →∗:D + A : I0/I = 1 + kqτ[A], (1) 𝐼⁄𝐼 =1+𝑘𝜏𝐴, (1) where I0 and I are ﬂuorescence intensities with and without a quenching agent, respectively, where I0 and I are fluorescence intensities with and without a quenching agent, re- kq is the constant of bimolecular quenching, τ is the lifetime of D* without a quenching spectively, kq is the constant of bimolecular quenching, τ is the lifetime of D* without a quenching agent molecule, and [A] is the concentration of a quenching agent. The τ value was found from the fluorescence decay curve for Tb(Phen). The lifetimes of the individual aqueous Tb(Phen) complexes are best fitted by biexponential curves (Figure S2). We used the average value of lifetime τav = 135 µs. The linear plot shape in the Stern–Volmer coordinates indicates that only one of the possible quenching mechanisms is in effect. The value of the bimolecular quenching constant calculated using Equation (1) is kq = 2.5 × 10 8 M−1·s−1. This quenching effect of the Tb3+ fluorescence was used to detect ascorbic acid. Figure 8 shows the calibration plot in the “fluorescence efficiency-ascorbic acid concentration” coordinates.

Figure 8. The calibration curve for ascorbic acid detection.

In the concentration range of 0.0016–0.19 mmol·L−1, the plot is linear. It is approximated by Equation (2):

Chemosensors 2021, 9, x FOR PEER REVIEW 8 of 11

Figure 7. Stern–Volmer plot for Tb(Phen) quenching by ascorbic acid.

According to current concepts of fluorescence quenching types, a possible process in this case can be dynamic quenching with emissionless transfer of electron excitation energy described by a Stern–Volmer equation (Equation (1)) for a bimolecular reaction 𝐷∗ + 𝐴→𝐷+𝐴∗:

𝐼⁄𝐼 =1+𝑘𝜏𝐴, (1) Chemosensors 2021, 9, 134 8 of 10 where I0 and I are fluorescence intensities with and without a quenching agent, respectively, kq is the constant of bimolecular quenching, τ is the lifetime of D* without a quenching agent molecule, and [A] is the concentration of a quenching agent. The τ value τ wasagent found molecule, from the and fluorescence [A] is the concentration decay curve offor a Tb(Phen). quenching The agent. lifetimes The ofvalue the individual was found aqueousfrom the Tb(Phen) fluorescence complexes decay curveare best for fitted Tb(Phen). by biexponential The lifetimes curves of the (Figure individual S2). We aqueous used Tb(Phen) complexes are best fitted by biexponential curves (Figure S2). We used the the average value of lifetime τav = 135 µs. average value of lifetime τ = 135 µs. The linear plot shape inav the Stern–Volmer coordinates indicates that only one of the The linear plot shape in the Stern–Volmer coordinates indicates that only one of the possible quenching mechanisms is in effect. The value of the bimolecular quenching con- possible quenching mechanisms is in effect. The value of the bimolecular quenching stant calculated using Equation (1) is kq = 2.5 × 10 8 M−1·s−1. constant calculated using Equation (1) is k = 2.5 × 10 8 M−1·s−1. This quenching effect of the Tb3+ fluorescenceq was used to detect ascorbic acid. Figure This quenching effect of the Tb3+ fluorescence was used to detect ascorbic acid. 8 shows the calibration plot in the “fluorescence efficiency-ascorbic acid concentration” Figure8 shows the calibration plot in the “fluorescence efficiency-ascorbic acid concentra- coordinates. tion” coordinates.

FigureFigure 8. 8. TheThe calibration calibration curvecurve for for ascorbic ascorbic acid acid detection. detection.

− InIn the the concentration concentration range range of of 0.0016–0.19 0.0016–0.19 mmol·L mmol·L−1, 1the, the plot plot is is linear. linear. It It is is approxi- approxi- matedmated by by Equation Equation (2): (2): y = 0.2617 + 28.936x (R2 = 0.99). (2) According to [24], the detection limit is correctly determined by the 3δ criterion for a linear dependence of the detected value on the concentration of a substance. According to this criterion, the detection threshold of a substance is determined by Equation (3):

Cmin = 3δ0/b, (3)

where δ0 is the standard deviation considered for measuring the signal in the blank experiment, b is the instrumental sensitivity coefﬁcient characterizing the change in the signal with the change in concentration, which is numerically equal to the tangent of the slope. In our case, the value δ0 = 0.71. −5 −1 The detection limit of ascorbic acid Cmin = 7.4 × 10 mol·L was found using Equations (2) and (3). To explore the practical application of Tb(Phen), the levels of ascorbic acid in tablet were measured. As shown in Table3, the recoveries of ascorbic acid in tablet samples were between 98.4% and 100.8%. The results demonstrate a good accuracy of ascorbic acid detection in pharmaceuticals using complex Tb(Phen) in micellar C12EO10 solution.

Table 3. Detection of ascorbic acid in tablets.

Samples Added, µM Detected, µM Recovery, % Tablet 1 0 not dedected - Tablet 2 50 52.1 ± 0.60 100.4 Tablet 3 100 ± 16 98.4 ± 0.91 98.4 Tablet 4 150 ± 0.09 151.2 ± 0.12 100.8 Chemosensors 2021, 9, 134 9 of 10

4. Conclusions In this paper, we studied the complexation of terbium(III) ions with 1,10-phenanthroline and ascorbic acid and the luminescent properties of the resulting complexes. It was shown that due to the formation of a mixed-ligand complex, the fluorescence of the terbium(III) ion is quenched. Quenching of excited states can be caused by any deactivation process that results from interactions of excited molecules with ascorbic acid. The linear plot shape in the Stern–Volmer coordinates is the indication of only one quenching mecha- nism in effect. For the first time, the effect of dynamic quenching was revealed for the mixed ligand Tb(Phen)-AA complex. The value of the bimolecular quenching constant is 2.5 × 108 M−1·s−1. The solubilizing efficiency of surfactants was estimated by the analysis of aggregation in micellar solutions with added complexes. It was revealed that the lifetime of the Tb(Phen)-AA complex depends on the type of surfactant. The longest lifetime is observed for micelles of a C12EO10 nonionic surfactant. The efficiency of fluorescence in micellar solutions depends on the acidity of the medium. Fluorescence reaches its maximum at the optimal pH values equal to 7–8. With optimal conditions set for the Tb(Phen) fluorescence, the quenching effect can be an effective tool for the molecular recognition of ascorbic acid with a detection limit of 7.4 × 10−5 mol·L−1. Facile preparation, commercial availability and high sensitivity of the proposed fluorescent probe make them prospective for detection bioactive substances.

Supplementary Materials: The following data are available online at https://www.mdpi.com/ article/10.3390/chemosensors9060134/s1, Figure S1: Fluorescence spectra of the Tb(Phen)-AA in microheterogeneous media: Tb(Phen)-AA-C12EO10; Tb(Phen)-AA-HTAB; Tb(Phen)-AA-SDS, Figure S2: Fluorescence decay curve of Tb(Phen) in aqueous solution. Author Contributions: Conceptualization, N.S. and Y.G.; methodology, N.S.; software, N.S.; vali- dation, N.S. and Y.G.; formal analysis, N.S.; investigation, N.S.; resources, N.S.; data curation, Y.G.; writing—original draft preparation, N.S.; writing—review and editing, N.S. and Y.G..; visualization, N.S. Both authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Russian foundation for basic research, grant number 20-03-00620. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Bünzli, J.-C.G. Lanthanide Luminescence for Biomedical Analyses and Imaging. Chem. Rev. 2010, 110, 2729–2755. [CrossRef] [PubMed] 2. Zhang, K.Y.; Yu, Q.; Wei, H.; Liu, S.; Zhao, Q.; Huang, W. Long-Lived Emissive Probes for Time-Resolved Photoluminescence Bioimaging and Biosensing. Chem. Rev. 2018, 118, 1770–1839. [CrossRef][PubMed] 3. Yi, Z.; Luo, Z.; Qin, X.; Chen, Q.; Liu, X. Lanthanide-Activated Nanoparticles: A Toolbox for Bioimaging, Therapeutics, and Neuromodulation. Acc. Chem. Res. 2020, 53, 2692–2704. [CrossRef][PubMed] 4. Ruiz-Medina, A.; Llorent-Martínez, E.J.; Ortega-Barrales, P.; Córdova, M.L.F.-D.; Pilar, O.-B. Lanthanide-Sensitized Luminescence as a Promising Tool in Clinical Analysis. Appl. Spectrosc. Rev. 2011, 46, 561–580. [CrossRef] 5. Srivastava, M.; Srivastava, N.; Mishra, P.; Malhotra, B.D. Prospects of nanomaterials-enabled biosensors for COVID-19 detection. Sci. Total. Environ. 2021, 754, 142363. [CrossRef] 6. Ortega-Algar, S.; Ramos-Martos, N.; Molina-Díaz, A. Flow-injection solid surface lanthanide-sensitized luminescence sensor for determination of p-aminobenzoic acid. Anal. Bioanal. Chem. 2008, 391, 715–719. [CrossRef] 7. Dong, Y.; Suo, H.; Yang, X.; Zhao, X.; Zhou, X. Eu(III) complex based on nonsteroidal anti-inﬂammatory drugs loxoprofen as the ligand: A novel low-toxic luminescent material for cell imaging. Spectrochim. Acta Part. A Mol. Biomol. Spectrosc. 2020, 229, 118014. [CrossRef] 8. Selivanova, N.; Vasilieva, K.; Galyametdinov, Y. Luminescent complexes of terbium ion for molecular recognition of ibuprofen. Luminescence 2013, 29, 202–210. [CrossRef] Chemosensors 2021, 9, 134 10 of 10

9. Shtykov, S.N.; Smirnova, T.D.; Bylinkin, Y.G.; Zhemerichkin, D.A. Fluorimetric determination of tetracyclines with the europium chelate of 1,10-phenanthroline in micellar solutions of anionic surfactants. J. Anal. Chem. 2005, 60, 24–28. [CrossRef] 10. Khaw, K.-T.; Bingham, S.; Welch, A.; Luben, R.; Wareham, N.; Oakes, S.; Day, N. Relation between plasma ascorbic acid and mortality in men and women in EPIC-Norfolk prospective study: A prospective population study. Lancet 2001, 357, 657–663. [CrossRef] 11. Güçlü, K.; Sözgen, K.; Tütem, E.; Ozyürek, M.; Apak, R. Spectrophotometric determination of ascorbic acid using copper(II)– neocuproine reagent in beverages and pharmaceuticals. Talanta 2005, 65, 1226–1232. [CrossRef] 12. Gökmen, V.; Kahraman, N.; Demir, N.; Acar, J. Enzymatically validated liquid chromatographic method for the determination of ascorbic and dehydroascorbic acids in fruit and vegetables. J. Chromatogr. A 2000, 881, 309–316. [CrossRef] 13. Zhang, L.-P.; Hu, B.; Wang, J.-H. Label-free colorimetric sensing of ascorbic acid based on Fenton reaction with unmodified gold nanoparticle probes and multiple molecular logic gates. Anal. Chim. Acta 2012, 717, 127–133. [CrossRef] 14. Kong, W.; Wu, D.; Li, G.; Chen, X.; Gong, P.; Sun, Z.; Chen, G.; Xia, L.; You, J.; Wu, Y. A facile carbon dots based fluorescent probe for ultrasensitive detection of ascorbic acid in biological fluids via non-oxidation reduction strategy. Talanta 2017, 165, 677–684. [CrossRef] 15. Smirnova, T.D.; Shtykov, S.N.; Zhelobitskaya, E.A. Energy transfer in liquid and solid nanoobjects: Application in luminescent analysis. Phys. Sci. Rev. 2018, 4, 4. [CrossRef] 16. Mohammadian, E.; Rahimpour, E.; Alizadeh-Sani, M.; Foroumadi, A.; Jouyban, A. An overview on terbium sensitized based- optical sensors/nanosensors for determination of pharmaceuticals. Appl. Spectrosc. Rev. 2020, 1–38. [CrossRef] 17. Tan, H.; Wu, J.; Chen, Y. Terbium(III) based coordination polymer microparticles as a luminescent probe for ascorbic acid. Microchim. Acta 2014, 181, 1431–1437. [CrossRef] 18. Wang, L.; Bian, G.; Dong, L.; Xia, Y.; Hong, S. Preparation of a Novel Fluorescence Probe of Terbium Composite Nanoparticles and its Application in the Determination of Ascorbic Acid. Microchim. Acta 2005, 150, 291–296. [CrossRef] 19. Selivanova, N.M.; Osipova, V.V.; Galyametdinov, Y.G. Effect of a lanthanide ion on the micellation and self-organization of lyotropic liquid crystal systems. Russ. J. Phys. Chem. A 2006, 80, 649–653. [CrossRef] 20. Fuguet, E.; Ràfols, C.; Roses, M.; Bosch, E. Critical micelle concentration of surfactants in aqueous buffered and unbuffered systems. Anal. Chim. Acta 2005, 548, 95–100. [CrossRef] 21. Selivanova, N.M.; Galeeva, A.I.; Gubaydullin, A.T.; Lobkov, V.S.; Galyametdinov, Y.G. Mesogenic and Luminescent Properties of Lyotropic Liquid Crystals Containing Eu(III) and Tb(III) Ions. J. Phys. Chem. B 2011, 116, 735–742. [CrossRef][PubMed] 22. Selivanova, N.M.; Konov, A.B.; Romanova, K.A.; Gubaidullin, A.; Galyametdinov, Y.G. Lyotropic La-containing lamellar liquid crystals: Phase behaviour, thermal and structural properties. Soft Matter 2015, 11, 7809–7816. [CrossRef][PubMed] 23. Smirnova, T.D.; Shtykov, S.N.; Nevryueva, N.V.; Zhemerichkin, D.A.; Parashchenko, I.I. Fluorimetric assay of flumequine using sensitized terbium fluorescence in organized media. Pharm. Chem. J. 2011, 44, 635–638. [CrossRef] 24. Eksperiandova, L.P.; Belikov, K.; Khimchenko, S.V.; Blank, T. Once again about determination and detection limits. J. Anal. Chem. 2010, 65, 223–228. [CrossRef]