Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Panjab 140001, IndiaCentre for Nanoscience & Nanotechnology, Panjab University, Chandigarh, Panjab 160014, IndiaFaculty of Chemistry, National Autonomous University of Mexico (UNAM), Mexico
r t i c l e i n f o
rticle history:eceived 25 October 2011eceived in revised form 16 January 2012ccepted 21 January 2012
a b s t r a c t
A dipodal receptor bearing imine linkages is decorated on the surface of ZnO nano-particles. The cationrecognition properties of pure dipodal receptor are compared with the assembly having same recep-tor decorated on ZnO. The remarkable changes in the recognition profile are established and the ZnOcoated with dipodal receptor system authenticates its use as a sensitive and selective sensor for Mg2+ in
vailable online 10 March 2012
eywords:hemosensoruantum dotanoparticlesnO
semi-aqueous medium. The ZnO coated dipodal receptor can be used to recognize the Mg2+ in cells ofSaccharomyces cerevisiae.
The development of chemosensors for the identification anduantification of important physiological and environmental ana-
ytes are of considerable importance [1–7]. The search is focusedor the high sensitivity, selectivity and multifunctionality of thesehemosensors for their effective operational usage [8–10]. Some-ime, selective chemosensors require long synthetic skills for theormulation of a receptor pseudocavity compatible with analyte11,12]. Moreover, receptor pseudocavity must be unique for thenalyte of interest and should not offer coordination sites for otherypes of analytes, not even for closely related i.e., selectivity for one
etal ion among other metal ions etc. Earlier, it is shown that theecoration of a non-selective receptor on the surface of CdSe/ZnSQDs) results in a nanocrystal hybrid, which has unique selectivityor two metal ions only through changes in UV-Vis Spectrum [13].his unique selectivity authenticates the use of this receptor sys-em for the simultaneous estimation of two metal ions. The theme
f this strategy is based upon the idea that a free organic recep-or is expected to be highly flexible and may adopt any geometryccording to the steric requirement of any metal ion. However, if
flexibility of this receptor is retarded, then this may improve theselectivity of the receptor. Under this strategy, the decoration oforganic receptor on the surface of nanoparticle may restrict someof the coordination modes; this will result into a relatively moreselective sensor [13]. The use of quantum dot as signaling unit iswell documented in literature [14–18]; only few reports are avail-able where the QDs are used as platform to anchor the organicligands.
Zinc oxide (ZnO) is a wide band gap semiconductor, havinghigh exciton binding energy of 60 meV and has stable wurtzitestructure [19,20]. It has attracted intensive research efforts forits unique properties and versatile applications in antireflec-tion coatings, transparent electrodes in solar cells, ultraviolet(UV) light emitters, diode lasers, piezoelectric devices, spin-electronics, surface acoustic wave propagator, antibacterial agent,photonic material and for sensing application [21]. In spite ofthese diverse applications, the use of ZnO as a platform toorganize non-selective receptor binding to an assembly, selec-tive for particular analyte is not explored. Although, similar toCdSe/ZnS (QDs) [13], the relatively cheaper ZnO decorated withnon-selective receptor may results into a nanocrystal hybrid, withunique selectivity for particular metal ion. Thus in continua-tion to the research interest of our research group [22–27], the
present investigation is designed to explore the possibility to dec-orate the imine linkaged dipodal receptor on the surface of ZnOnanoparticles to fine tune the recognition properties of resultantsensor.
firm that the magnitude of hydrogen bonding in the assembly 2 islesser than the one prevailed in the pure compound 1. Secondly, the
OH signal splitting into two signals with �ı = 0.9 ppm, confirm-ing that two OH protons are in different environment. The same
N
ON
O
NO
H
H
HN
N
N
OH
OH
HO
QD
N
N
HO
OH
68 H. Sharma et al. / Sensors and A
. Experimental
.1. Materials and methods
Chemicals were purchased from Sigma Aldrich and were usedithout further purification. The NMR spectra were recorded onvance-II (Bruker) instrument, which operated at 400 MHz for 1HMR. IR spectra were recorded on a Bruker Tensor 27 spectrometer
or the compounds in the solid state as KBr discs. The absorp-ion spectra were recorded on a Specord 250 Plus Analytikjenapectrophotometer. The fluorescence measurements were per-ormed on a Perkin Elmer L55 Fluorescence spectrophotometer.he fluorescence microscopy was performed on ZEISS Axiovert 200Inverted microscope) with PALM optical tweezers cum Micro Dis-ection system. Images with resolution of 320 × 240 pixels wereecorded by camtasia recorder at 12 frames/s. The particle size ofg2+ complex of 2 was measured by using external probe feature ofetrohm Microtrac Ultra Nanotrac Particle Size Analyzer (Dynamic
ight Scattering).
.2. Synthesis of sensor 2
The sensor 2 was synthesized by coating 1 on the surface ofnO. The compound 1 was synthesized by taking Ethylene diamine60 mg, 1.0 mmol) and Salicylaldehyde (305 mg, 2.5 mmol) in dry
ethanol. A yellow product was separated out and this productas washed with methanol and the product was characterized with
H NMR and IR spectroscopy. The ZnO was synthesized by mixinghe alcoholic solution of Zn(NO3)2·6H2O (595 mg, 2.0 mmol) withlcoholic solution of NaOH (120 mg, 3.0 mmol). A white productas separated out and the product was washed with ethanol. Theroduct was dried at 150 ◦C. The ZnO was characterized with TEMnd X-ray diffraction pattern. The compound 1 (804 mg, 3 mmol)as taken along with ZnO (100 mg) in dry CHCl3 and the solutionas refluxed for 15 h. The progress of the reaction was monitoredith IR and UV–vis absorption spectroscopy by taking a small sam-le from the reaction mixture. Upon completion of reaction a lightellow colored product was separated with filtration. The productas washed with chloroform and dried under vacuum. The finalroduct was characterized with TEM, 1H NMR and IR spectroscopy.
.3. Metal binding studies
All the recognition studies were performed at 25 ± 1 ◦C andefore recording any spectrum a sufficient time was give withhaking to ensure the uniformity of the solution. The cation bind-ng ability of 1–2 was determined by mixing standard solutionsf host (10 �M) along with a fixed amounts of particular metalitrate salt (50 �M) in HEPES buffered in DMSO/H2O (7:3, v/v). Theation recognition behavior of 1–2 was evaluated from the changesn fluorescence spectrum of sensor upon addition of a particular
etal salt. The fluorescence spectra of 1–2 were recorded withxcitation wavelengths shown in respective figures. The titrationas performed by taking standard solution of sensor 2 (10 �M)
long with successive addition of magnesium nitrate (0–50 �M) inEPES buffered DMSO/H2O (7:3, v/v). The quantum yields of recep-
or 1–2 and complexation constants of 1 with different metal ionsere calculated using literature methods. To evaluate any possi-
le interference due to different cations for the estimation of Mg2+,olutions were prepared containing 2 (10 �M) and Mg2+ (50 �M)
long both with and without other interfering metal ions (50 �M)n DMSO/H2O (7:3, v/v) HEPES buffered solution (pH 7.0 ± 0.1).he size distributions of complex formed between 2 and Mg2+ inMSO/H2O (7:3, v/v) system (with variable time of complexation
ors B 166– 167 (2012) 467– 472
maintained by dropwise addition of Mg2+ to 2) were measuredusing Dynamic Light Scattering (DLS) based particle size analyzer.
2.4. Cell culture
The culture medium was taken in culture tubes guarded withplug. For inoculation, the dry microbes were dissolved in water andone drop of this suspension was added to each tube of medium.The cultures were a clear solution (to naked eye) at the beginningand with the growth of microbes, the cloudiness appeared. Theculture tubes were marked as A, B, C, D; where tubes A and C con-tains normal culture medium; while tubes B and D were enrichedwith Mg2+ (0.1 M). All the four tubes were placed in incubator at27 ◦C. After incubation, the content of each tube was centrifugedand washed three times with double distilled water. The contentof tubes C and D was treated with sensor 2 and again content wascentrifuged and washed with DMSO/H2O (7:3, v/v) solvent com-bination. Finally, the microbes of all the tubes were submitted forfluorescence microscopy and tubes A and D for SEM images.
3. Results and discussion
3.1. Synthesis and characterization of sensor 2
The receptor 1 and ZnO were synthesized by literature pro-cedures [28,29], the X-ray diffraction pattern of ZnO showingscattering angles (2�) of 31.7, 34.4, 36.2, 47.5, 56.6, 62.8, and 67.9corresponding to the reflection from 1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0,1 0 3 and 1 1 2 crystal planes, respectively (Fig. S1). The 1H NMRspectrum of 1 is showing signals at ı 2.8 (s, 4H, CH2), 6.8 (m,4H, Ar), 7.3 (d, 2H, Ar), 7.4 (d, 2H, Ar), 8.6 (s, 2H, CH N ), 13.4(s, 2H, OH). The receptor 1 was coated on the surface of ZnO byrefluxing the two in dry CHCl3 (Scheme 1). The progress of the reac-tion was monitored with IR and UV–vis absorption spectroscopythrough monitoring the band of imine linkages. It was found thatwith time the absorbance due to imine linkages increased and ulti-mately become constant after 12 h.
The exact nature of binding sites of 1 responsible for coordi-nation with ZnO were determined from the changes in the 1HNMR spectrum of 1 upon interaction with ZnO. The OH signalsof pure 1 at 13.4 ppm specify that these protons are in the toneof strong hydrogen bonding. The decoration of 1 on QD leads toan upfield shift and splitting of this signal; the upfield shifts con-
1 2
Scheme 1. Synthesis of 1 and 2.
H. Sharma et al. / Sensors and Actuators B 166– 167 (2012) 467– 472 469
F in DMa g bands ges o
taswndqri1shoigf
3
3sgmTbobnsb
F(D
ig. 1. Family of spectra: (A) Partial 1H NMR spectra of pure receptor 1 (recordedssembly 2 (recorded in DMSO-d6); (C) Partial IR spectra of pure receptor 1 showinhowing splitting in band for CH N; (E) TEM images of pure ZnO and (F) TEM ima
ype of splitting in signals was observed for proton of imine link-ges ( CH N ) and aromatic unit i.e., signal for imine linkage wasplitted with �ı = 0.15 ppm and each aromatic proton was splittedith �ı = 0.03–0.2 ppm. The splitting of each signal into two sig-als of equal intensity signifies that one half of 1 is in a completelyifferent chemical environment than the other half part. This isuite possible if one half of 1 attaches with the ZnO and other halfemains free (Scheme 1). Similarly, the band due to imine linkagesn IR spectrum of 1, also splits into two parts upon interaction of
with ZnO. The TEM image of pure ZnO is showing the uniformize of nanoparticles. As evident from Fig. 1E and F, that pure ZnOave no interaction with each other. On the other hand, the sizef 2 is increased to an appreciable extent. This can be due to thentermolecular hydrogen bonding between the CH N and OHroup; thus in this way, several units of 2 assembled together toorm a bigger particle size.
.2. Optimization of the responsive condition of sensor 2 for Mg2+
The UV–vis absorption spectrum of pure ZnO exhibited a peak at60 nm corresponding to the exciton state of ZnO. The absorptionpectrum of ZnO coated with 1, showed a band due to strong intrali-and O(phenolato) → CN(imino) charge transfer transition in theolecule [30,31]; this band superimposed the exciton peak of ZnO.
he fluorescence spectrum of pure ZnO exhibited a green emissionand at 580 nm; however this band was quenched upon cappingf ZnO with receptor 1. The noticed effect can be explained on the
asis of model offered by Dijken et al. [32]; according to which, theano-particle surface plays an important role in the visible emis-ion. A recombination center (V∗∗
O ) is formed, when the valenceand hole is trapped by surface states and then tunnels back into
ig. 2. (A) Changes in fluorescence intensity of 2 (10 �M) upon addition of 50 �M of a pexcitation at 355 nm); (B) normalized fluorescence intensity (Io − I/Io) of 2 (10 �M) at 48MSO/H2O (7:3, v/v); the reported values represents the out-comings of three independe
SO-d6); (B) Partial 1H NMR spectra of receptor 1 upon its decoration on ZnO or for CH N; (D) Partial IR spectra of 1 upon its decoration on ZnO (or assembly 2)
f ZnO coated with 1 (i.e., assembly 2).
oxygen vacancies containing one electron (V∗O). This recombina-
tion of a shallowly trapped electron with a deeply trapped hole ina V∗∗
O center is responsible for visible emission. In uncapped ZnOnanoparticles, existence of huge surface defects results in the visi-ble emission, as shown in Fig. S2. Out of the oxygen and zinc defects(dangling bonds) at the grain boundaries; the oxygen defects mayfacilitate the binding with hydroxyl group of 1 by hydrogen bond-ing. Furthermore, the zinc defects at the interface adsorb oxygen,which can also combine with hydroxyl and imine linkages of 1. As aresult, the interface defects decrease, and the probability of surfacetrapped hole is decreased. At the same time, 1 spatially block theprocess of surface trapped hole tunneling back into the particlesto form V∗∗
O center. Both effects lead to decreasing the probabilityof V∗∗
O recombination (visible emission). Therefore, in comparisonwith the uncapped ZnO nanoparticles, the visible emission of ZnOnanoparticles capped with 1 becomes very weak.
The emission band (�max = 480 nm) of 2 (10 �M) recorded inHEPES buffered DMSO/H2O (7:3, v/v) was red shifted as compareto the emission band observed in solid state fluorescence spectrumof 2, however both excited at the same wavelength (�ext = 355 nm).A similar type of band was observed in the fluorescence spectrumof 1; indicating that the observed band is due to the emission fromimine linkages (Fig. S3). The effect of pH on the fluorescence of2 was determined by fluorescence titration under different pH ofsolution from 10.7 to 3.2. However, at low pH (∼4.0), the fluo-rescence intensity quenched, most likely due to the breakage ofimine linkages, thus decreasing the concentration of fluorophore.
Interestingly, high pH (∼9.0) switched “On” the fluorescence of 2.This is due to deprotonation of the phenolic hydroxyl group whichotherwise quenches the fluorescence by vibrationally coupling theexcited state to water [33]. Therefore, working at pH 7.0 ensures the
articular metal nitrate salt in HEPES buffered DMSO/H2O (7:3, v/v) solvent system0 nm upon addition of 5.0 equiv. of a particular metal nitrate salt in HEPES bufferednt determinations.
470 H. Sharma et al. / Sensors and Actuators B 166– 167 (2012) 467– 472
0
100
200
300
400
500
600
700
800
900
1000
360 385 410 43 5 46 0 48 5 51 0 53 5 56 0 585
Flu
ore
sen
ce In
ten
sit
y
Wavelength (nm)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5 10 15 20 25
No
rmali
zed
flu
ore
sen
ce in
ten
sit
y
Conc. Of Mg2+
(micromol.)
A B
F f Mg2+
o concev
ia(i(taamNciwh21sms
3
flwitbTtwotdcnbdmroisTaF
system
To investigate the applications of 2 as a chemosensor for Mg2+
recognition in biological system, a microbe (Saccharomyces cere-
ig. 3. (A) Fluorescence spectra changes of 2 (10 �M) upon addition of nitrate salt of normalized fluorescence intensity (Imin − I/Imin − Imax) of 2 (10 �M) at 445 nm vs.alues represents the out-comings of three independent determinations.
mine linkages remains intact for participate in ion-binding. Thusll the metal binding studies of receptor 2 were conducted at pH 7.0±0.1) maintained with HEPES buffer. The changes in fluorescencentensity of 2 upon addition of other metal salts were recordedFig. 2A) and it was found that the addition of Mg2+ salt (50 �M)o the solution of compound 2 (10 �M) resulted in enhancementnd blue shift in emission band at 480. Under the same conditionss used for Mg2+, fluorescence response of 2 was tested for otheretal ions such as Na+, K+, Ba2+, Sr2+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+,i2+, Cu2+, Zn2+ and are shown in Fig. 2A; no significant fluores-ence change of 2 occurred in the presence of (50 �M) other metalons. The quantum yields of receptor 2 towards different metal ions
ere calculated using literature methods [34–36] (Table S1). Theighest quantum yield was found for the complex formed between
and Mg2+. Similar investigations were performed with receptor; the quantum yields of receptor 1 with various metal ions arehown in Table S1 and complexation constants of 1 towards variousetal ions are shown in Table S2. The results revealed no significant
electivity for any particular metal ion.
.3. Selective and sensitive detection of Mg2+ using sensor 2
To learn more about the properties of 2 as a receptor for Mg2+,uorescence titration was carried out. All the titration experimentsere carried out in DMSO/H2O (7:3, v/v) by adding aliquots of metal
on. The addition of increasing amounts of Mg2+ from (0 to 50 �M)o the solution of compound 2 resulted in a increase in emissionand at 445 nm, which was attributed to 1.Mg2+ complex (Fig. 3).he successive increase of intensity at 445 nm may authenticatehe use of this receptor for estimation of Mg2+ and a linear relationas obtained between the fluorescence intensity of 2 as a function
f concentration of Mg2+ from 5 to 21 �M. It is interesting to notehe �max in the fluorescence spectra shown in Figs. 2 and 3; i.e., theegree of blue shift induced by Mg2+ addition under two differentonditions. The addition of 5 equiv. of Mg2+ to the solution of 2 pro-ounced more blue shift. The observed behavior is explained on theasis that the shifts in the �max upon complexation of 2 with Mg2+
epend upon the binding sites responsive for the coordination ofetal ion. The PET is ‘ON’ from the OH, and if any metal ion bind to
eceptor by using the OH binding site; this will switch ‘ON’ the flu-rescence intensity on the same wavelength. However, if any metalon binds to CH N i.e., a close binding to fluorophore leads to the
hift in �max due to the modulations in charge transfer transitions.he metal binding test shown in Fig. 2A conclude less shift in �max
s compare to the shifts observed during the titration shown underig. 3A. Thus the time taken for the mixing of 2 and Mg2+ seems
(0–50 �M) in HEPES buffered DMSO/H2O (7:3, v/v) (excitation at 355 nm); (B) plotntration of Mg2+ (5–21 �M) in HEPES buffered DMSO/H2O (7:3, v/v); the reported
to influence the fluorescence signature. This can be explained asfollow: (a) The stepwise addition of Mg2+ to 2 leads to the for-mation of intramolecular coordination sphere (shown in Fig. S7)using both OH and CH N linkages, thus PET is cancelled from
OH and CT band is also affected. (b) A single lot addition of Mg2+
to 2; leads to the formation of intermolecular coordination sphere(shown in Fig. S7) using OH binding sites, which are projectedoutside. Thus this leads to the cancellation of PET only; hence thereis no shift in �max. Our belief regarding the use of intermolecularand intramolecular binding is confirmed with DLS studies. The sizedistribution of complex formed between 2 and Mg2+ in DMSO/H2O(7:3, v/v) system with variable time of complexation maintained bydropwise addition of Mg2+ to 2 are shown in Fig. S6. A regular trendwas obtained i.e., as the time of mixing of Mg2+ with 2 increased;the small sized complex is formed and vice versa. This is only pos-sible if single lot addition of Mg2+ to 2 lead intermolecular bindingand step-wise addition restrict to intramolecular binding as shownin Fig. S7.
To test the practical applicability of compound 2 as a Mg2+ selec-tive fluorescence sensor; competitive experiments were carriedout in the presence of Mg2+ (5 equiv.) mixed with one of Na+, K+,Ba2+, Sr2+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+ (5 equiv.).As shown in Fig. 4, no significant variation in the intensity ratio(I480/I355) was found by comparing the profile with and withoutthe other metal ions, means that assembly 2 has a high selectivityfor Mg2+.
3.4. Evaluation of sensor 2 for Mg2+ recognition in biological
Fig. 4. Estimation of Mg2+ in the presence of other metal ions (Na+, K+, Ba2+, Sr2+,Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+) in HEPES buffered DMSO/H2O (7:3, v/v).
H. Sharma et al. / Sensors and Actuators B 166– 167 (2012) 467– 472 471
Fig. 5. Microscopic images of: (A) microbe cells cultured in normal medium, (B) microbe cells cultured in medium enriched with Mg2+, (C) microbe cells cultured in normalm Mg2+
m fore ps
vma2fw
m((MTbcmfiSMtd
4
otrtMflmsa
edium and treated with 2, (D) microbe cells cultured in medium enriched withicrobe cells and (F) microbe cells cultured with Mg2+ and treated with sensor 2. Be
olvent mixture.
isiae) was cultured in normal broth and secondly in experimentaledia containing Mg2+. The cells cultured in normal broth as well
s cultured in a media containing Mg2+ were treated with sensor dissolved in a DMSO/H2O (7:3, v/v) solvent mixture. Before per-orming microscopy observations, the microbe cells were washedith a DMSO/H2O (7:3, v/v) solvent mixture.
The microscopy images taken of (a) blank microbe cells, (b)icrobe cells cultured in medium enriched with Mg2+ (0.1 M),
c) microbe cells cultured in normal broth and treated with 210 �M), and (d) microbe cells cultured in medium enriched with
g2+(0.1 M) and treated with sensor 2 (10 �M) are shown in Fig. 5.he microscopic investigations revealed that sensor 2 is capable ofinding Mg2+ in a cellular medium. The microscopic image (Fig. 5D)learly shows that the sensor passed through the membrane of theicrobe and stained the cytoplasm enriched with Mg2+. To con-
rm that the sensor did not lead to destruction of the microbes,EM images of normal microbes and microbe cells cultured withg2+ and treated with sensor 2 were obtained. The smoothness of
he microbe surface observed in both cases confirms that the sensorid not cause any damage on the surface of the microbe.
. Conclusion
A new strategy is designed to enhance the selectivity of a flu-rescent sensor by retarding the flexibility of sensor; which leadso restrict some of the coordination modes, thus resulted into aelatively more selective sensor. The sensor was found to be selec-ive for Mg2+ among all the tested metal ions, the coordination of
g2+ within the pseudo-cavity of sensor leads to enhancement of
uorescence intensity of receptor based on the cancellation of PETechanism and modulation of charge transfer transitions. The sen-
or offered an interesting opportunity to study the Mg2+ content in biological system i.e., cytoplasm of S. cerevisiae.
and treated with 2; SEM images showing the surface morphology of: (E) normalerforming microscopy, the microbe cells were washed with a DMSO/H2O (7:3, v/v)
Acknowledgments
This work was supported by Indo-Mexican Joint Research Poject[DST/INT/MEX/01-04/2011(iii)] and ISIRD grant from IIT Ropar.Authors are thankful to SAIF Panjab University Chandigarh for NMRSpectra and to Dr. Inderpreet Kaur of CSIO Chandigarh for help influorescence microscopy.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.snb.2012.01.076.
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Biographies
Hemant Sharma obtained M.Sc. (Chemistry) degree from Panjab University, Chandi-garh and currently working as a PhD student in chemistry department of IIT Ropar.His area of research includes study of photophysical and biological applications ofimine linked and nanoparticles based receptors.
Navneet Kaur received PhD (in 2005) from National Institute of PharmaceuticalEducation and Research (NIPER) Mohali. Currently, she is an Assistant Professorof Nanoscience & Nanotechnology at Panjab University, Chandigarh. Her researchinterests include the Medicinal and Materials Chemistry. She has published 30research papers in reputed international journals.
Thangarasu Pandiyan obtained Ph.D. (in 1993) from Bharathidasan University,India. He has published 38 articles in national and international research journals.Currently, he is professor in National Autonomous University of Mexico (UNAM),Mexico.
Narinder Singh obtained PhD (in 2006) degree from Guru Nanak Dev University,
Amritsar. After spending 3 years as Postdoctoral fellow, he joined Indian Instituteof Technology Ropar (IIT Ropar) as Assistant Professor of Inorganic Chemistry. Cur-rently, he is the Coordinator, Chemistry Department of IIT Ropar. He has publishedmore than 40 research papers in international peer reviewed journals. His area ofresearch includes Supramolecular and Materials Chemistry.
