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Lifetime and Fluorescence Quantum Yield of Two Fluorescein-Amino Acid-Based Compounds in Different Organic Solvents and Gold Colloidal Suspensions

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Lifetime and Fluorescence Quantum Yield of Two Fluorescein-Amino Acid-Based Compounds in Different Organic Solvents and Gold Colloidal Suspensions chemosensors Article Lifetime and Fluorescence Quantum Yield of Two Fluorescein-Amino Acid-Based Compounds in Different Organic Solvents and Gold Colloidal Suspensions Viviane Pilla 1,2,*, Augusto C. Gonçalves 1,3, Alcindo A. Dos Santos 3 and Carlos Lodeiro 1,4 ID 1 BIOSCOPE Group, LAQV-REQUIMTE, Chemistry Department, Faculty of Science and Technology, University NOVA of Lisbon, 2829-516 Caparica, Portugal; [email protected] (A.C.G.); [email protected] (C.L.) 2 Instituto de Física, Universidade Federal de Uberlândia-UFU, Av. João Naves de Ávila 2121, 38400-902 Uberlândia, Brazil 3 Instituto de Química, Universidade de São Paulo-USP, Av. Prof. Lineu Prestes 748, CxP 26077, 05508-000 São Paulo, Brazil; [email protected] 4 Proteomass Scientific Society, Rua dos Inventores, Madan Park, 2829-516 Caparica, Portugal * Correspondence: [email protected] Received: 11 May 2018; Accepted: 28 June 2018; Published: 30 June 2018 Abstract: Fluorescein and its derivatives are yellowish-green emitting dyes with outstanding potential in advanced molecular probes for different applications. In this work, two fluorescent compounds containing fluorescein and an amino acid residue (compounds 1 and 2) were photophysically explored. Compounds 1 and 2 were previously synthesized via ester linkage between fluorescein ethyl ester and Boc-ser (TMS)-OH or Boc-cys(4-methyl benzyl)-OH. Studies on the time-resolved fluorescence lifetime and relative fluorescence quantum yield (φ) were performed for 1 and 2 diluted in acetonitrile, ethanol, dimethyl sulfoxide, and tetrahydrofuran solvents. The discussion considered the dielectric constants of the studied solvents (range between 7.5 and 47.2) and the photophysical parameters. The results of the lifetime and φ were compared with those obtained for compounds 1, 2 and fluorescein without an amino acid residue in alkaline aqueous solutions. Moreover, as a preliminary result compound 2 (S-cysteine) was tested in the presence of gold nanoparticles as an aggregation colorimetric probe. Keywords: fluorescein; lifetime; fluorescence quantum yield; gold nanoparticles 1. Introduction One of the most commonly yellowish-green emitting dyes used for the preparation of advanced molecular probes applied in biological, toxicological, biomedical, and environmental studies is fluorescein [1–4]. It is a very versatile dye due to its attractive photophysical properties, such as high extinction coefficients, high fluorescence quantum yield (φ), biocompatibility and low cost [5–7]. Fluorescein and its derivatives can be found as differently charged species depending on the pH of the aqueous solution. The range of these species progresses through the protonated cation form (acidic + − 2− solution, FH3 ) to the neutral species (FH2) and then to the anionic (FH ) and the dianionic (F ) entities in alkaline solutions [5,8,9]. As a result of the electron distribution around the fluorescein core, these different entities possess unique photophysical properties that affect the absorbed and emitted light. The other photophysical parameters, such as the quantum yield and the lifetimes of the excited states, are also closely related to the pH [9,10], polarity [11,12], and hydrogen bonding power (HBP) [13,14] of the Chemosensors 2018, 6, 26; doi:10.3390/chemosensors6030026 www.mdpi.com/journal/chemosensors Chemosensors &1$2, 6, x FOR PEER REVIEW , o& 11 'epe"'ing o" the pH o& the aAueous so%utio". The r!"(e of these spe$ies pro(resses +¿¿ throu(h the proto"!te' $!tio" &orm 4!$i'i$ so%utio", FH 3 5 to the "eutr!% spe$ies Q ,Q 4#H,5 !"' the" to the !"io"i$ 4#H 5 !"' the 'i!"io"i$ 4# 5 entities i" !%Caline solutions [5,8,9]. As ! result o& the ele$tro" 'istributio" arou"' the fluorescein $ore, these 'ifferent entities possess unique photophysic!% properties that !Fe$t the !+sor+e' !"' emitte' light3 The other photophysic!% parameters, su$h as the Auantum yie%' !"' the lifetimes o& the e@$ite' states, are also $losely relate' to the pH [9,10], polarity [11,12], !"' hy'ro(e" +o"'ing poEer 4HBP5 [13,14] o& the $hemic!% e")ironment J-L3 The so%)ent polarity 'etermines the eAuilibrium form o& fluorescein !"' its 'erivatives; for example, in polar protic so%)ents, 'isso$iatio" o& the !$idic pheno% hy'ro(e" (pR! S 03< in Eater5 le!'s to !" eAuilibrium +etEee" the neutr!% !"' anio"ic &orms; i" $ontrast, the pre'ominant form in aprotic so%)ents is the neutr!% %!$tone J-L3 To $har!$terize the so%)ato$hromism, fluorescein Eas extensively explore' usin( !+sorption, ste!'y-state, !"' time-reso%)e' fluoresce"$e spe$troscopy [6,11,15–18]. The !+sorptio"Oemissio" +!"' shifts exhibite' ! high 'epe"'e"$e o" the HBP !"' 'iele$tric $onstants o& the so%)ents [13,14,19]. ConseAuently, the so%)ent-depe"'ent photophysic!% $har!$terizatio" o& neE $hromophores must +e performe' Eith 'iffering so%)ents [20,21] to est!+lish Chemosensors 2018, 6, 26 2 of 11 'iverseness to suit a variety of potential applications. #%uores$ei" in its free-!$i' &orm is a )ery )ers!ti%e Guoro(e"i$ bui%'i"( +%o$C for chemical environment [8]. The solvent polarity determines the equilibrium form of fluorescein and the sy"thesisits derivatives; o& "eE for example, mo%e$u%!r in polar proticpro+es solvents, J,,L3 dissociation HoEe)er, of the 'ue acidic to phenol the hydrogen prese"$e o& o@y(e"!te'(pKa = or(!"i$ 6.4 in water) &u"$tio"!% leads to an equilibrium(roups o)er between the the $ore neutral o& and the anionic mo%e$u%e, forms; in Guores$ei" contrast, is poor%y these%e$ti)e predominant &or form ! spe$iB$ in aprotic solvents !"!%yte3 is the neutralThe outermost lactone [8]. To characterize(roups the!rou"' solvatochromism, the @!"the"e fluorescein was extensively explored using absorption, steady-state, and time-resolved fluorescence moiety spectroscopy$!" +e stru$tur!%%y [6,11,15–18]. The mo'iBe' absorption/emission usi"( +iomo%e$u%es band shifts exhibited 4su$h a high !s dependence !mi"o on!$i's or pepti'es5the HBPto and!'Pust dielectric the constants $hromophore of the solvents properties [13,14,19]. Consequently, to suit the solvent-dependent 'esire' !"!%yti$!% !pp%i$!tio"photophysical J<,,,L3 characterization of new chromophores must be performed with differing solvents [20,21] Re$entto establish stu'ies diverseness h!)e to suithighlighte' a variety of potential the potential applications. o& fluorescein as ! fluorescent Fluorescein in its free-acid form is a very versatile fluorogenic building block for the synthesis of $ompou"'new molecular &or fu"$tionalizing probes [22]. However, (o%' due to"!"oparticles the presence of oxygenated (Au-*Ps), organic !"' functional preparatio" groups o& smart materials.over the core ofSe)er!% the molecule, applications fluorescein is h!)e poorly +ee" selective reporte' for a specific as analyte. i" sensitive The outermost 'ete$tio" o& toxinsgroups J,;L1 around in the)itro xanthene !"' moiety in )ivo can be structurally+reast $!"$er modified imaging using biomolecules J,<L1 (suchthe as'ete$tio" amino o& immu"o(lo+ulinacids or peptides) G to J,/L1 adjust the! 'ual-mo'e chromophore properties imaging to suit system the desired J,0L1 analytical sele$tive application re$o(nitio" [4,22]. Recent studies have highlighted the potential of fluorescein as a fluorescent compound for !"' Auantitativefunctionalizing 'ete$tio" gold nanoparticles o& thioure! (Au-NPs), andJ,?L1 preparation tumor of suppressor smart materials. (p/;5 Several 'ete$tio" applications using time-reso%)e'have been reportedfluoresce"$e as in sensitive J,-L1 detection intr!$ellular of toxins [23 ];pHin vitro mappin(and in vivo !"'breast $ellular cancer pH measurementimaging [24u"'er]; the detection'ru( stimulatio" of immunoglobulin J,.L1 G!" [25 optic!%]; a dual-mode mer$ury imaging $hemosensor system [26]; J;=L1 selective recognition and quantitative detection of thiourea [27]; tumor suppressor (p53) detection using !"' othertime-resolved "!"oar$hite$ture fluorescence [28]; intracellular materials pH mapping &or and'ru( cellular 'elivery pH measurement [31]. under With drug these applicationsstimulation in [mind,29]; an optical Ee ha)e mercury 'esigne' chemosensor !"' [30]; andsynthesize' other nanoarchitecture tEo $ompou"'s, materials for drug $ !"' & 4#iguredelivery 1) +y [31 !]. Withsimple these applicationsesterifi$atio" in mind, were!$tio" have designed o& the and synthesizedphenolic two portio" compounds, Eith the prote$te'1 and amino2 (Figure 1!$i's) by a simple Bo$-Ser esterification 4TB reactionMS5-OH of the phenolic!"' Bo$-Cys portion with 4<-methy%-be">y%5-OH, the protected amino acids Boc-Ser (TBDMS)-OH and Boc-Cys (4-methyl-benzyl)-OH, respectively. Both compounds, 1 and respe$ti)e%y32 were explored Both $ompou"'s, recently by us, as probes$ !"' for & the Eere trivalent e@p%ore' ions (Al3+, re$e"t%y Fe3+, Ga3+, and+y Crus,3+) as!s well pro+es as &or the tri)!%e"tfor Hg2+ io"s[22]. 4A%;T, #e;T, G!;T, !"' Cr;T5 !s Ee%% !s &or H(,T J,,L3 O O O Si O O O S O O O O O O O HN O HN O O O 1 2 Figure 1. Chemical structure of compounds 1 and 2. In order to explore the interaction with different organic solvents and colloidal suspensions, the present work reports the solvent effect on the UV–vis absorption, steady-state fluorescence, and time-resolved fluorescence spectroscopy, and the relative fluorescence quantum yield, φ, of compounds 1 and 2. These studies were performed in four different solvents, acetonitrile, dimethylsulfoxide (DMSO), tetrahydrofuran (THF), and absolute ethanol. For fluorescein without an amino acid residue, the spectroscopic, lifetime, and φ results in alkaline aqueous solutions are reported for comparison. To evaluate the effects of the presence of a sulfur atom in 2, its interaction with gold nanoparticles in an ethanolic colloidal suspension was also performed.
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