Fluorescence Quenching of Polycyclic Aromatic Hydrocarbons byNitromethane within Ionic Liquid Added Aqueous Anionic MicellarSolutionShruti Trivedi and Siddharth Pandey*
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
ABSTRACT: Applicability of nitromethane as selective fluorescencequenching agent for discriminating between alternant versus nonalternantpolycyclic aromatic hydrocarbons (PAHs) is examined for eight PAHsdissolved in aqueous micellar sodium dodecyl sulfate (SDS) media in theabsence and presence of varying wt % of water-miscible ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). AlternantPAHs follow quenching sphere-of-action model, whereas nonalternantPAHs demonstrate simple linear Stern−Volmer type quenching behavior.Nitromethane quenched fluorescence emission of both the alternant andnonalternant PAHs dissolved in aqueous SDS micellar media where theselectivity of the nitromethane as the quencher is lost. However, thenitromethane begins to retain its selectivity in quenching fluorescence fromalternant PAHs over that from the nonalternant PAHs as soon as smallamount of IL [bmim][BF4] (ca. 0.5 wt %) is added to anionic SDS solution.The dual role of IL is demonstrated where initially at lower concentrations it works as an electrolyte helping to establishquenching selectivity of nitromethane and at higher concentrations as a cosolvent where quenching of nonalternant PAHfluorescence again starts to become significant due to the changes in the physicochemical properties of the micellar assemblies.Use of IL as an effective additive to the aqueous anionic micelle media for PAH separation and analysis is amply demonstrated.
■ INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) are well-knownpotent atmospheric pollutants constituting the largest class ofchemical carcinogens and mutagens.1−3 They have beenclassified as priority pollutants by US Environmental ProtectionAgency (EPA) and are becoming an increasing concern due totheir significant human exposure.4 Because of their resistance tobiodegradation, they are very persistent in the environment.1,2,5
The presence of these persisted PAHs in environments maylead to plethora of problems which includes cancer and geneticmutation. High prenatal exposure to PAHs results in lower IQand childhood asthma. The detrimental properties of PAHscoupled to growing awareness of environmental pollutionprompted researchers to develop analytical methods to isolatethe individual organic compounds of this class.Among many classifications, one way to categorize the PAHs
is to separate them as alternants and nonalternants. AlternantPAHs have fully conjugated aromatic systems (Figure 1). Ifeach carbon atom in the aromatic structure is labeled,alternately skipping an atom between labels, then alternantPAHs have a structure such that no two atoms of the same typeare adjacent. Whereas, nonalternant PAHs (Figure 2) do nothave conjugated aromaticity and have a structure in which suchlabeling results in two adjacent atoms of the same type. Thesestructural modifications can bring about great changes in thephysicochemical and optical properties. In order to identify,separate, and detect alternant and nonalternant PAHs,
Received: December 4, 2012Revised: January 7, 2013Published: January 7, 2013
fluorescence spectroscopy offers high sensitivity and selectivitybecause the quantum yield of most PAHs is quite significant.Zander,6−9 Acree,10−19 and McGuffin20−23 groups have shownthat nitromethane selectively quenches the fluorescence ofalternant PAHs as opposed to the fluorescence of nonalternantPAHs. In a series of publications, Acree and co-workers usingseveral alternant and nonalternant PAHs have amplyestablished the nitromethane selective quenching rule.10−19,24
Anionic micellar solutions are used as media for PAHseparation in micellar electrokinetic capillary chromatography(MEKC). Terabe and co-workers accounted that PAHs couldbe separated by cyclodextrin-modified MEKC.25 Later, Cole’sgroup separated PAHs by MEKC using Bile salt in the presenceof organic modifier.26 Fu et al. utilized MEKC with aqueousorganic solvent to separate PAHs and established that MEKCcondition can be optimized by altering the alcohol content, theconcentration of anionic surfactant sodium dodecyl sulfate(SDS), and the separation temperature.27 However, Acree andco-workers found that selectivity of nitromethane asfluorescence quencher for alternant PAHs over nonalternantPAHs was lost when anionic micellar solutions were used assolubilizing mediawithin anionic micellar solutions nitro-methane started to quench the fluorescence from nonalternantPAHs as well.13,24
Because of their inherent nonvolatility and uniquephysicochemical properties, interest in ionic liquids (ILs) hasgrown dramatically in the past decade.28−34 Owing to theirpotential environmentally benign nature, ILs are utilized inconcert with other environmentally friendly substances such aswater,35−37 glycol family solvents,38−40 polymer solutions,41,42
supercritical fluids,43−45 and surfactant-based systems,46−49
among others. In this context, efforts have been endowed byresearchers to study the behavior of ILs in modifying theproperties of surfactant-based systems.50−58 ILs were found todemonstrate unique role in altering the properties ofsurfactant/micellar solutions.54,56,57 It is observed that atlower concentrations ILs behave similar to electrolytes due totheir strong electrolytic dissociation, whereas at higherconcentrations, ILs act as cosolvents when added to aqueoussurfactant solutions.56,57 Behera et al. reported the dualbehavior of IL 1-butyl-3-methylimidazolium tetrafluoroborate([bmim][BF4]) in altering important properties of aqueousSDS solution.57 At lower concentrations (i.e., ≤2 wt %), therole of IL appeared to be similar to that of an electrolyte asbmim+ cations locate close to the micellar surface, thus partlyneutralizing anionic micellar charged surface. However, athigher concentration (2 wt % < [bmim][BF4] ≤ 30 wt %), thedissociation of IL decreases, and it behaves more like a polarcosolvent.57
To explore the potential of ILs as additive during separationprocesses, in this paper we present fluorescence quenchingbehavior of nitromethane toward select alternant and non-alternant PAHs when dissolved in aqueous anionic micellarSDS solution in the presence of water-miscible IL [bmim]-[BF4]. We report the important finding that addition of[bmim][BF4] to micellar SDS solution helps nitromethaneretain its selectivity in quenching fluorescence of alternantPAHs as opposed to that of nonalternant PAHs. IL-addedaqueous anionic micellar solutions as possible media for PAHanalysis is established from these studies.
■ EXPERIMENTAL SECTION
Materials. Anthracene (99%), perylene (99+%), fluoran-thene (99%), and benzo[b]fluoranthene were purchased fromAldrich and were used as received. Pyrene and benzo[j]-fluoranthene were obtained in highest purity from Sigma-Aldrich Co. and SUPELCO, respectively, and stored underdried conditions. Benzo[a]pyrene and indeno[1,2,3-cd]pyrenewere purchased from Accustandard and stored under driedconditions. IL [bmim][BF4] was obtained from Merck (highpurity, halide content <100 ppm, water content <100 ppm) andwas stored under dry argon. The solvent used to preparesurfactant solutions in this work is the doubly distilleddeionized water which was obtained from a Millipore Milli-QAcademic water purification system having ≥18 MΩ·cmresistivity. Ethanol (99.9% purity) and dichloromethane, usedto prepare PAH stock solutions, were purchased from Merckand Sigma-Aldrich Co., respectively. SDS was purchased fromSISCO Research Laboratories. Nitromethane (99+%) pur-chased from Acros Organics was used as the quenching agent.
Methods. The required amount of PAH was weighed toprepare stock solution using a Denver Instrument balance witha precision of ±0.1 mg. The stock solution of various PAHs wasprepared in ethanol and dichloromethane and stored in amberglass vial in dark at 4 ± 1 °C to retard any photochemicalreactions. An appropriate amount of PAH solution from thestock was transferred to a glass vial. The ethanol ordichloromethane was evaporated with a gentle stream ofhigh-purity nitrogen gas, and 0.1 M aqueous SDS solution wasadded to achieve the desired final concentration of the probe.Appropriate aliquots of [bmim][BF4] were then added byweight to the above solution to obtain required concentrationof [bmim][BF4] in 0.1 M aqueous SDS solution. Nitromethanewas subsequently added as the fluorescence quencher.A Varian Cary 100-Bio double-beam spectrophotometer with
variable bandwidth was used for the acquisition of the UV−vismolecular absorbance spectra. Fluorescence spectra wereacquired on model FL 3-11, Fluorolog-3 modular spectro-fluorometer with single Czerny−Turner grating excitation andemission monochromators having 450 W Xe arc lamp as theexcitation source and PMT as the detector. The fluorimeter waspurchased from Horiba−Jobin Yvon, Inc. All data wereacquired using 4 mm or 1 cm path length quartz cuvettes.Spectral response from appropriate blanks was subtractedbefore data analysis. Data analysis was performed by SigmaPlotv10 software. In order to obtain the fluorescence lifetime of thePAH, excited-state intensity decay data were acquired in thetime-domain using HORIBA Jobin Yvon, Inc., Fluorocubetime-correlated single photon counting (TCSPC) fluorimeter.The PAH dissolved in 0.1 M aqueous SDS solution (with orwithout IL) at room temperature was excited at 340 nm using aUV-pulsed NanoLED-340 source having pulse width <1.0 nsand at 405 nm using NanoLED-405 violet laser diode. Theemission was collected using Peltier-cooled red-sensitive TBX-04 PMT detection module at desired wavelengths. The datawere collected with a DAQ-MCA-3 Series (P7882) multi-channel analyzer. The instrument response function (IRF) wasobtained using a scattering solution of glycogen in water(glycogen from bovine liver, Type IX, Aldrich). The excited-state intensity decays were analyzed using DAS6 analysissoftware and were fitted to single/double-exponential decaymodels.
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261819
Fluorescence emission intensities were corrected for primaryinner-filtering artifacts and self-absorption arising from theabsorption of excitation radiation ( fprim) by the nitromethaneand the PAH solute, respectively, according to a simplifiedexpression:
= ≈fFF
10 Aprim
corr
obs0.5
(1)
where Fcorr and Fobs refer to the corrected and observedfluorescence emission intensities, respectively, and A is theabsorbance/cm of path length at the excitation wavelength.
■ RESULTS AND DISCUSSIONThe lowest unoccupied molecular orbital (LUMO) of analternant PAH, which gets occupied after excitation, is usuallyhigher in energy than the LUMO of the quencher nitromethane(Scheme 1).7−9,14,20,21 As a result, electron/charge transfer isenergetically favorable between nitromethane and the excitedalternant PAH, resulting in a decrease in the fluorescenceintensity. In nonalternant PAHs, on the contrary, the LUMOwhich is occupied after excitation is either of comparable orlower energy than that of nitromethane, making the electron/charge transfer less feasible. Therefore, there is slight or nochange in the fluorescence intensity of nonalternant PAHs inthe presence of nitromethane. The extent of quenching can bealtered by merely changing the electronic character of thesurrounding environment/milieu in order to either stabilize ordestabilize the partial positive charge that temporarily developson the excited PAH during the electron/charge donation tonitromethane. Micellar media, in this context, have provided aconvenient means to introduce ionic character around thecybotactic region of the excited PAH, thus altering thenitromethane quenching efficiency. In this regard, anionicmicellar media is proposed by the Acree group to stabilize the
partial positive charge that develops on excited PAHs,subsequently allowing the fluorescence quenching of evennonalternant PAHs to occur by nitromethane, whereas thecationic and nonionic micellar media tend to inhibit thequench i n g o f fluo r e s c en c e f r om nona l t e r n an tPAHs.10,12,13,17,18,24 The selectivity in fluorescence quenchingof alternant PAHs as opposed to nonalternant PAHs bynitromethane was observed to be lost within anionic micellarmedia whereas it was retained in cationic and nonionic micellarsolutions.12,13,18,24 We now present the outcomes of IL[bmim][BF4] addition on nitromethane quenching efficiencyof select alternant and nonalternant PAHs dissolved in aqueousanionic micellar solution of SDS.
Alternant PAHs. The effect of nitromethane addition onfluorescence emission spectra of an alternant PAH, anthracene,dissolved in 0.1 M aqueous SDS in the absence and presence of10 wt % [bmim][BF4] is presented in Figure 3. We have foundthat fluorescence emission of all four alternant PAHs isquenched by nitromethane irrespective of the presence of[bmim][BF4]. Figure 4 presents F0/F versus [CH3NO2] foralternant PAHs dissolved in 0.1 M aqueous SDS in thepresence of varying wt % [bmim][BF4] (0−30 wt %) underambient conditions. (Since the presence of micellar aggregatesis an essential condition for the anionic headgroup charge toaffect the nitromethane quenching mechanism, the [SDS] wastaken well above its critical micellar concentration [cmc].)Here, F0 and F are the fluorescence intensities of PAHs in theabsence and presence of nitromethane, respectively. Interest-ingly, F0/F versus [CH3NO2] plots for all wt % of[bmim][BF4] show distinct upward curvature, implying thatthe alternant PAH−nitromethane fluorophore−quencher pairin aqueous SDS does not show simple Stern−Volmer behaviorin the presence of [bmim][BF4].
59,60 If dynamic or static
Scheme 1. Simplified Molecular Orbital Diagram Indicating Favorable Conditions for Electron Transfer between ElectronDonor Excited Alternant PAH and an Electron Acceptor Quenching Agenta
aThe dotted line represents the potential of a reference electrode.
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261820
quenching alone were operative, linear behavior between F0/Fand [CH3NO2] would be expected.Positive deviations from the Stern−Volmer equation usually
occur when the extent of quenching is large. Perrin introducedthe concept of an “active sphere” for rigid solutionsa volumeof interaction around a quencher molecule such that thefluorophore excited within this molecule is instantaneouslyquenched.61 It implies that the fluorophore and quencher donot actually form a ground-state complex; instead, it seems thatthe apparent static component is due to the quencher beingadjacent to the fluorophore at the moment of excitation. Theseweakly associated fluorophore−quencher pairs show weak orno fluorescence and as a result appear to be dark complexes.Frank and Vavilov modified this model by combining the Perrinmodel with a dynamic quenching one.62 This model is
endowed with a good description of quenching within solutionsof high viscosity with short-range electron/charge-exchangeinteraction, with an active sphere radius of ∼10−15 Å.63 Themodified form of the Stern−Volmer equation is given by eq 2
= +FF
K(1 [CH NO ])eV0D 3 2
[CH NO ]3 2
(2)
where V is the volume of the quenching sphere-of-action andKD is the dynamic (collisional) quenching constant. All thecurves for alternant PAHs in Figure 4 represent fits to thisquenching sphere-of-action model. The recovered parametersare collected in Table 1. It is clear that the quenching behavioris well described by a sphere-of-action model for anthracene,benzo[a]pyrene, and pyrene, and is fair-to-good for the case ofperylene.A careful examination of the quenching sphere-of-action radii
(R, calculated from V, Table 1) reveals no clear trend.Irrespective of the presence of [bmim][BF4] or the identity ofthe PAH, the radii are statistically very similar. These radiicorrespond to almost first solvation shell of the excited PAH,and if nitromethane is within the sphere-of-action of these radii,there exists a unit probability that quenching occurs beforethese molecules diffuse apart. The slender differences in theradii (e.g., slightly larger for perylene and slightly smaller forbenzo[a]pyrene) could be attributed to the different extent ofthe first solvation shells of different PAHs. For micellar SDS,distribution of nitromethane and PAHs within the solutioncould be assumed to be fairly similar.The recovered KD (Table 1), on the other hand, apart from
depending on the identity of the PAH, depends heavily on theamount of [bmim][BF4] added. For the four alternant PAHs,the KD in the absence of [bmim][BF4] decreases in the orderpyrene > benzo[a]pyrene > anthracene > perylene. This orderis in agreement with the ionization potential of these PAHs (orthe gap between the LUMOs of the PAHs and thenitromethane) and confirms what is reported in the literatureearlier.64,65 Interestingly, however, KD decreases for all alternantPAHs as a small amount of [bmim][BF4] is added to micellarSDS solution, suggesting decrease in the efficiency of collisional
Figure 3. Quenching of fluorescence emission of anthracene (10 μM)by nitromethane dissolved in 0.1 M aqueous SDS in the absence(panel A) and presence (panel B) of 10 wt % IL [bmim][BF4] atambient conditions (λex = 357 nm).
Figure 4. F0/F versus [CH3NO2] plots showing nitromethane quenching of the fluorescence from alternant PAHs dissolved in aqueous SDS +[bmim][BF4] mixtures at ambient conditions. Solid curves represent fit to a quenching sphere-of-action model (eq 2) (recovered parameters arereported in Table 1).
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261821
(dynamic) quenching. This outcome is attributed in major partto the screening of the anionic micellar surface charge bybmim+, thus reducing the stabilization of partial positive chargethat generates on excited PAH during the electron/chargetransfer as PAHs are known to solubilize in the palisade layerclose to the micellar surface.66−70 Minor contribution todecreased KD from increase in the viscosity of the milieu inwhich fluorescence quenching is taking place should not beignored.71,72 At higher concentration of IL (i.e., 10−30 wt %),the changes in the properties of the micellar aggregates start toplay a role where aggregation number (Nagg) decreases alongwith decrease in micellar size.56,57 This leads toward thequenching behavior observed in polar isotropic media wherealternant PAHs are quenched significantly by nitromethane. Itis interesting to note that for all four alternant PAHs the KD arefound to be minimum when 5 wt % [bmim][BF4] is added tomicellar SDS. All in all, KD decreases nearly 3-fold foranthracene and pyrene and nearly 4-fold for perylene andbenzo[a]pyrene in the presence of 30 wt % [bmim][BF4].Bimolecular quenching rate constants (kq) were calculated
for the alternant PAHs dissolved in aqueous micellar SDSsolution in the absence and presence of [bmim][BF4] using
τ=
⟨ ⟩k
Kq
D
0 (3)
Here, ⟨τ0⟩ is the average fluorescence lifetime of the PAH in theabsence of quencher. The fluorescence lifetimes for alternantPAHs in aqueous SDS micellar solution with varyingconcentration of [bmim][BF4] were measured in the absenceof quencher. Figure 5 depicts representative excited-stateintensity decay curves of alternant PAHs in aqueous micellarSDS. Similarly, decay curves were also collected for [bmim]-[BF4] added aqueous SDS solution. While the intensity decaysof all alternant PAHs satisfactorily fit to a single-exponentialdecay model for [bmim][BF4] ≤ 0.5 wt %, double-exponentialdecay scheme was needed to fit the data for [bmim][BF4] > 0.5wt %. This is expected in a complex media such as[bmim][BF4] added aqueous micellar SDS solution whereheterogeneity in PAH distribution sites is expected. Estimated⟨τ0⟩ from recovered τ’s and kq thus calculated are presented inTable 1. Fluorescence lifetimes of PAHs show no clear trend as[bmim][BF4] is added to micellar SDS solutionit increasesfor anthracene, decreases for benzo[a]pyrene and pyrene, andremains same for perylene as [bmim][BF4] concentration isincreased. kq, on the other hand, follows a trend similar to KD
and, in the absence of [bmim][BF4], reaches close to diffusion-controlled value. This is expected for favorable quenching ofalternant PAHs by nitromethane within aqueous anionicmicellar SDS media.10,12,13,17,18
Nonalternant PAHs. Figure 6 presents fluorescence spectraof nonalternant PAH fluoranthene in the absence and presence
Table 1. Modified Parameters of Alternant PAHs/Nitromethane “Sphere-of-Action” Quenching within Aqueous SDS +[bmim][BF4]
wt % [bmim][BF4] KD (M−1) ⟨τ⟩ (ns) kq (109 M−1 s−1) V (M−1) R (Å) r2
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261822
of 10 wt % [bmim][BF4] as the quencher nitromethane isadded to 0.1 M aqueous SDS solution. Figure 7 presents [(F0/F) −1] versus [CH3NO2] for all concentrations of [bmim]-[BF4] studied for all four nonalternant PAHs (fluoranthene,benzo[b]fluoranthene, benzo[j]fluoranthene, and indeno[1,2,3-cd]pyrene). It is clear from Figure 7 that quenching of fournonalternant PAHs by nitromethane follows a simple linearStern−Volmer relationship59 irrespective of the identity of thenonalternant PAH or the presence of IL [bmim][BF4]:
τ− = = ⟨ ⟩FF
K k1 [CH NO ] [CH NO ]0SV 3 2 q 0 3 2 (4)
where KSV is the Stern−Volmer quenching constant, whichbecomes KD in case of dynamic (collisional) quenching. Thequenching mechanism of nonalternant PAHs by nitromethaneis different from that of the alternant PAHs. We suspect thatsince the electron/charge transfer process is energetically notfavored in nonalternant cases (see quenching mechanism inScheme 1), the quenching perhaps follows a simpler Stern−Volmer relationship with no transient effects. From decrease inthe fluorescence lifetime in the presence of quencher, we foundthe quenching of nonalternant PAHs by nitromethane in[bmim][BF4]-added aqueous micellar SDS solutions to bedynamic in nature. Recovered KD are presented in Table 2along with kq that were estimated from fluorescence lifetimes ofnonalternant PAHs in the absence of quencher nitromethane.(Figure 8 depicts the excited-state intensity decay curves ofnonalternant PAHs dissolved in aqueous micellar SDS.Similarly, other decay curves were collected for [bmim][BF4]added aqueous SDS solutions.) Similar to the case of alternantPAHs, intensity decay of nonalternant PAHs for [bmim][BF4]> 0.5 wt % best fit to a double-exponential decay model.A careful examination of the entries in Table 2 reveals several
important outcomes. As expected and revealed by the KD, thefluorescence of all four nonalternant PAHs is quenched bynitromethane in micellar SDS media in the absence of[bmim][BF4] showing the loss of quenching selectivity asshown by the Acree group earlier.13,24 Most interestingly, KDand kq are decreased significantly as a small amount of[bmim][BF4] (0.5 wt %) is added, clearly showing nitro-methane to retain its selectivity in quenching fluorescence ofalternant PAHs over that of nonalternant PAHs in SDS micellarmedia. This is attributed to the fact that for lower ILconcentrations screening of anionic micellar surface charge by
Figure 5. Excited-state intensity decay data of alternant PAHs dissolved in aqueous SDS at ambient conditions. The blue curves denote theinstrumental response function (IRF) measured using a dilute glycogen suspension. The top panels provide single-exponential fits to experimentaldata, and the lower panels show weighted residuals of the corresponding fits. Excitation is achieved using 340 or 405 nm Nano LEDs.
Figure 6. Quenching of fluorescence emission of fluoranthene (10μM) by nitromethane dissolved in 0.1 M aqueous SDS in the absence(panel A) and presence (panel B) of 10 wt % IL [bmim][BF4] atambient conditions (λex = 380 nm).
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261823
[bmim]+ helps nitromethane to retain its selectivity utilizing theelectrolytic behavior of IL at lower concentrations. (In fact,quenching is reduced for both alternant and nonalternant;however, reduction for nonalternants is so high that the
selectively is retained.) The developing partial positive chargeon excited-nonalternant PAH is no longer stabilized due to thescreening of micellar negative charge by [bmim]+. Furtheraddition of [bmim][BF4] results in small increase in quenchingof nonalternant PAHs. This is again attributed to the alteredmicellar properties of SDS at higher concentrations of[bmim][BF4]. kq for nonalternant class are 101−103 timeslower than the alternant class due to the unfavorable electron/charge transfer process between nonalternant PAH and thequencher. This leads to restricted quenching even in thepresence of anionic SDS micelles. Lower kq for nonalternantPAHs indicates that the process is not diffusion-controlledonly; the charge/electron transfer is simply energetically notthat favored as reported by researchers earlier.7−9,14,20,21
■ CONCLUSIONS
Nitromethane is known to quench the fluorescence of alternantPAHs but not the fluorescence of nonalternant PAHs whendissolved in polar protic and polar aprotic isotropic organicmedia. This establishes the selectivity of nitromethane as afluorescence quencher to discriminate between alternant andnonalternant PAHs. When dissolved in aqueous anionicmicellar media, the fluorescence of nonalternant PAHs is alsoquenched by nitromethane and the selectivity is lost due to thestabilization of the partial positive charge by negative anionicmicellar surface that develops on the excited PAH during theelectron/charge transfer. Addition of small amounts of IL[bmim][BF4] to aqueous anionic micellar media helpsnitromethane retain its quenching selectivity as the fluorescencequenching of nonalternant PAHs is again inhibited due to thepartial neutralization of the anionic micellar surface charge bythe presence of bulky bmim+ cations of the IL as at lowerconcentrations; IL dissociates to good extent within themicellar media. However, at higher IL concentrations, due tothe changes in the physicochemical properties of the micellarmedia as IL now acts as a cosolvent, the quenching ofnonalternant PAHs again starts to become prominent.Alternant PAHs appear to follow quenching sphere-of-actionmodel whereas nonalternant PAHs follow simple linearquenching behavior according to Stern−Volmer equation.
Figure 7. [(F0/F) − 1] versus [CH3NO2] plots showing nitromethane quenching of nonalternant PAHs dissolved in aqueous SDS + [bmim][BF4]mixtures at ambient conditions. Solid lines represent fit to a linear Stern−Volmer model (eq 4) (recovered parameters are reported in Table 2).
Table 2. Parameters of Nonalternant PAHs/NitromethaneQuenching within Aqueous SDS + [bmim][BF4] (Eq 4)
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261824
The dual role of IL is highlighted where initially it works as anelectrolyte assisting in quenching selectivity and later as acosolvent. The outcomes of this investigation establish ILs asadditive to aqueous anionic micellar media for PAH separationand analysis.
NotesThe authors declare no competing financial interest.
■ REFERENCES(1) Lee, M. L.; Novotny, M. V.; Bartle, K. D. Analytical Chemistry ofPolycyclic Aromatic Compounds; Academic Press: New York, 1981.(2) Dinh, T. V. Chemical Analysis of Polycyclic Aromatic Compounds;Wiley: New York, 1988.(3) McGuffin, V. L.; Goodpaster, J. V. In Encyclopedia ofEnvironmental Analysis and Remediation; Meyers, R. A., Ed.; Wiley:New York, 1998; pp 3815−3831.(4) Richard, A. M.; Woo, Y. Mutat. Res. 1990, 242, 285−303.(5) Dipple, A. Polycyclic Hydrocarbons and Carcinogenesis. InProceedings of ACS Symposium Series 283; Harvey, R. G., Ed.; AmericanChemical Society: Washington, DC, 1985; pp 1−18.(6) Tucker, S. A.; Darmodjo, H.; Acree, W. E., Jr.; Fetzer, J. C.;Zander, M. Appl. Spectrosc. 1992, 46, 1260−1265.(7) Breymann, U.; Dreeskamp, H.; Koch, E.; Zander, M. Chem. Phys.Lett. 1978, 59, 68−70.(8) Dreeskamp, H.; Koch, E.; Zander, M. Z. Naturforsch. 1975, 30A,1311−1314.(9) Zander, M.; Breymann, U.; Dreeskamp, H.; Koch, E. Z.Naturforsch. 1977, 32A, 1561−1563.(10) Pandey, S.; Fletcher, K. A.; Powell, J. R.; McHale, M. E. R.;Kauppila, A.-S. M.; Acree, W. E., Jr.; Fetzer, J. C.; Dai, W.; Harvey, R.G. Spectrochim. Acta, Part A 1997, 53, 165−172.
(11) Amszi, V. L.; Cordero, Y.; Smith, B.; Tucker, S. A.; Acree, W. E.,Jr.; Yang, C.; Abu-Shaqara, S.; Harvey, R. G. Appl. Spectrosc. 1992, 46,1156−1161.(12) Pandey, S.; Acree, W. E., Jr.; Fetzer, J. C. Mikrochim. Acta 1998,129, 41−45.(13) Pandey, S.; Acree, W. E., Jr.; Fetzer, J. C. Anal. Chim. Acta 1996,324, 175−181.(14) Acree, W. E., Jr.; Pandey, S.; Tucker, S. A.; Fetzer, J. C. PolycyclicAromat. Compd. 1997, 12, 71−123.(15) Pandey, S.; Acree, W. E., Jr.; Cho, B. P.; Fetzer, J. C. Talanta1997, 44, 413−421.(16) Pandey, S.; Acree, W. E., Jr.; Scott, L. T.; Necula, A.; Fetzer, J.C.; Mulder, P. P. J.; Lugtenburg, J.; Cornelisse, J. Polycyclic Aromat.Compd. 1999, 13, 79−92.(17) Pandey, S.; Fletcher, K. A.; Acree, W. E., Jr.; Fetzer, J. C.Fresenius J. Anal. Chem. 1998, 360, 669−674.(18) Pandey, S.; Powell, J. R.; Acree, W. E., Jr.; Cho, B. P.; Kum, J.;Yang, C.; Harvey, R. G. Polycyclic Aromat. Compd. 1997, 12, 1−19.(19) Tucker, S. A.; Griffin, J. M.; Acree, W. E., Jr. Analyst 1994, 119,2129−2133.(20) Ogasawara, F. K.; Wang, Y.; McGuffin, V. L. Appl. Spectrosc.1995, 49, 1−7.(21) Chen, S. H.; Evans, C. E.; McGuffin, V. L. Anal. Chim. Acta1991, 246, 65−74.(22) Goodpaster, J. V.; Harrison, J. F.; McGuffin, V. L. J. Phys. Chem.A 2002, 106, 10645−10654.(23) Howerton, S. B.; Goodpaster, J. V.; McGuffin, V. L. Anal. Chim.Acta 2002, 459, 61−73.(24) Pandey, S.; Acree, W. E., Jr.; Fetzer, J. C. J. Lumin. 1997, 71,189−197.(25) Terabe, S.; Miyashita, Y.; Shibata, O.; Barnhart, E. R.; Alexander,L. R.; Patterson, D. G.; Karger, B. L.; Hosoya, K.; Tanaka, N. J.Chromatogr. 1990, 516, 23−31.(26) Cole, R. O.; Sepaniak, M. J.; Hinze, W. L.; Gorse, J.; Oldiges, K.J. Chromatogr. 1991, 557, 113−123.(27) Fu, X.; Lu, J.; Chen, Y. Talanta 1998, 46, 751−756.(28) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792−793.(29) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003.
Figure 8. Excited-state intensity decay data of nonalternant PAHs dissolved in aqueous SDS at ambient conditions. The blue curves denote theinstrumental response function (IRF) measured using a dilute glycogen suspension. The top panels provide single-exponential fits to experimentaldata, and the lower panels show weighted residuals of the corresponding fits. Excitation is achieved using a 340 nm Nano LED.
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261825
(30) Malhotra, S. Ionic Liquid Applications: Pharmaceuticals,Therapeutics, and Biotechnology; ACS Symposium Series; AmericanChemical Society: Washington, DC, 2010.(31) Freemantle, M. An Introduction to Ionic Liquids, 1st ed.; RSCPublishing: Cambridge, UK, 2009.(32) Koel, M., Ed.; Ionic Liquids in Chemical Analysis; CRC Press:Boca Raton, FL, 2009.(33) Brennecke, J. F.; Rogers, R. D.; Seddon, K. R. Ionic Liquids IV:Not Just Solvents Anymore; ACS Symposium Series; AmericanChemical Society: Washington, DC, 2007.(34) Rogers, R. D.; Seddon, K. R., Eds.; Ionic Liquids as GreenSolvents:Progress and Prospects; ACS Symposium Series 856; AmericanChemical Society: Washington, DC, 2003.(35) Fletcher, K. A.; Pandey, S. Appl. Spectrosc. 2002, 56, 266−271.(36) Pandey, S.; Fletcher, K. A.; Baker, S. N.; Baker, G. A. Analyst2004, 129, 569−573.(37) Trivedi, S.; Malek, N. I.; Behera, K.; Pandey, S. J. Phys. Chem. B2010, 114, 8118−8125.(38) Sarkar, A.; Trivedi, S.; Baker, G. A.; Pandey, S. J. Phys. Chem. B2008, 112, 14927−14936.(39) Sarkar, A.; Trivedi, S.; Pandey, S. J. Phys. Chem. B 2008, 112,9042−9049.(40) Trivedi, S.; Pandey, S. J. Phys. Chem. B 2011, 115, 7405−7416.(41) Singh, P.; Pandey, S. Green Chem. 2007, 9, 254−261.(42) Chen, J.; Spear, S. K.; Huddleston, J. G.; Rogers, R. D. GreenChem. 2005, 7, 64−82.(43) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. J. Am. Chem. Soc.2002, 124, 10276−10277.(44) Solinas, M.; Pfaltz, A.; Cozzi, P. G.; Leitner, W. J. Am. Chem. Soc.2004, 126, 16142−16147.(45) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F.Nature 1999, 399, 28−29.(46) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33−36.(47) Merrigan, T. L.; Bates, E. D.; Dorman, S. C.; Davis, J. H., Jr.Chem. Commun. 2000, 2051−2052.(48) Friberg, S. E.; Yin, Q.; Pavel, F.; Mackay, R. A.; Holbrey, J. D.;Seddon, K. R.; Aikens, P. A. J. Dispersion Sci. Technol. 2000, 21, 185−197.(49) Gao, Y.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B.X.; Hou, W. G.; Li, G. Z. Green Chem. 2006, 8, 43−49.(50) Rai, R.; Pandey, Sh.; Baker, S. N.; Vora, S.; Behera, K.; Baker, G.A.; Pandey, S. Chem.Eur. J. 2012, 18, 12213−12217.(51) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.;Armstrong, D. W. Chem. Commun. 2003, 2444−2445.(52) He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666−12667.(53) Atkin, R.; Warr, G. G. J. Phys. Chem. B 2007, 111, 9309−9316.(54) Behera, K.; Kumar, V.; Pandey, S. ChemPhysChem 2010, 11,1044−1052.(55) Behera, K.; Om, H.; Pandey, S. J. Phys. Chem. B 2009, 113, 786−793.(56) Behera, K.; Pandey, S. Langmuir 2008, 24, 6462−6469.(57) Behera, K.; Pandey, S. J. Phys. Chem. B 2007, 111, 13307−13315.(58) Rai, R.; Baker, G. A.; Behera, K.; Mohanty, P.; Kurur, N. D.;Pandey, S. Langmuir 2010, 26, 17821−17826.(59) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;Kluwer Academics/Plenum Publishers: New York, 2006.(60) Valeur, B. Molecular Fluorescence: Principles and Applications;Wiley-VCH Verlag GmbH: Weinheim, 2002.(61) Perrin, F. C. R. Acad. Sci. Paris 1924, 178, 1978−1988.(62) Frank, J. M.; Vavilov, S. I. Z. Phys. 1931, 69, 100−110.(63) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970.(64) Kuroda, H. Nature 1964, 201, 1214−1215.(65) Harada, M.; Ohga, Y.; Watanabe, I.; Watarai, H. Chem. Phys.Lett. 1999, 303, 489−492.(66) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133−138.(67) Cardinal, J. R.; Mukerjee, P. J. Phys. Chem. 1978, 82, 1614−1620.
(68) Pyter, R. A.; Ramachandran, C.; Mukerjee, P. J. Phys. Chem.1982, 86, 3206−3210.(69) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979,101, 279−291.(70) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88,1098−1101.(71) Arik, M.; Celebi, N.; Onganer, Y. J. Photochem. Photobiol., A2005, 170, 105−111.(72) Eftink, M. R.; Ghiron, C. A. J. Phys. Chem. 1976, 80, 486−493.
The Journal of Physical Chemistry C Article
dx.doi.org/10.1021/jp311897m | J. Phys. Chem. C 2013, 117, 1818−18261826
