ARTICLE pubs.acs.org/JPCC
Intriguing Fluorescence Behavior of Diiminic Schiff Bases in the Presence of in situ Produced Noble Metal Nanoparticles Mainak Ganguly,† Anjali Pal,‡ and Tarasankar Pal*,† † ‡
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India
bS Supporting Information ABSTRACT: Diiminic Schiff bases (DSBs) with a variety of spacers in between the two imine bonds have been prepared from salicylaldehyde and different diamines. Gold and silver nanoparticles (NPs) are photoproduced as stable hydrosol from the respective metal salts in the presence of alkaline DSBs. In solution, the capping capability of DSBs renders stability toward both the NPs. The phenomenon of capping of noble metal by DSBs is gifted with surprising alteration of the fluorescence property of DSBs. Varying concentration of the local field on the fluorophore as well as the coupling between the molecular dipole of the fluorophore and the surface plasmon band of the metal have been ascribed to be the key point of these stunning phenomena in solution phase.
’ INTRODUCTION Salen and salen-like molecules are diiminic Schiff bases (DSBs) obtained from salicylaldehyde and diamines. The chemistry of salen has become a subject of much attention nowadays because of the multifaceted applications. “Salen” is now an eloquent vocabulary in synthetic chemistry. After Pfeiffer1 reported “salen” in the year 1933, it has traversed a long journey. In analytical, biomedical, as well as synthetic chemistry, salen and salen derivatives have been exploited immensely. Diehl and Hach2 have demonstrated the cobalt complex of salen as a potential oxygen carrier. Chiral salarg ligand is synthesized following a similar route to that of achiral salen. Fluorescence spectra of both the ligands and their metal complexes are studied, confirming that salen-like ligands are potential fluorophores and eventually become probe molecules.3 Liu et al.4 reported Mn-salen as a promising probe molecule to detect traces of DNA in solution because of the prominent alteration of the fluorescence property of the molecule. Remarkable decrement of fluorescence intensity is noticed for Mn-Schiff bases bound to DNA, showing a blue shift of excitation and emission peaks. This observation relates to the observed hypochromic shift in the UV absorption spectra. Salicylideneaniline-based organogelator was synthesized with very high fluorescence quantum yield owing to J aggregation of the molecule and inhibition of intramolecular rotation in the gel state.5 Massimo et al.6 demonstrated the metal perturbed ligand centered state of a novel salen-like compound with 12 carbon atoms bridged between an iminic nitrogen atom. There are also some other reports where the fluorescence properties of salen, salen derivatives, and their metal complexes are exploited for the determination of a trace amount of hazardous and useful substances, r 2011 American Chemical Society
for example, Mg,7 H 2 O2 , triacetone organic peroxides, 8 and cyanide. 9 Aoki et al. 10 determined aliphatic primary amines by flow injection fluorometry using beryllium-Schiff base complexes. A well announced word in fluorescence spectroscopy is selfquenching. Fluorescein11 and other xanthene type dyes12 are well-studied due to their self-quenching property with remarkably low quantum yield at high probe concentration. With the help of the fluorescence quenching strategy involving some proximal dyes, mechanistic aspects of protein folding dynamics have been developed. Zhuang et al.13 demonstrated that unfolded titin molecule is capable of folding in its native state while tagged with some dyes. The high fluorescence intensity of the dye died down due to the close proximity of the dye. A study of the characteristics of self-quenching of fluorescence of lipid conjugated rhodamine in membrane was done by MacDonald.14 There is an interesting report by Munkholm et al.15 for self-quenching of fluoresceinamine. Conversion of amine to amide renders restoration of fluorescence, indicating the relationship of the donor ability of the atom and turning off or on of the fluorophore. The mechanism of self-quenching is very diverse and is altered with the alteration of the system. Cross relaxation between fluorophore pairs,16,17 exciton migration to trap sites,18 exciton exciton recombination,19 and dimer and excimer formation20 23 have been stated as the probable reasons. Received: May 27, 2011 Revised: October 10, 2011 Published: October 10, 2011 22138
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Figure 1. Structures of the six DSBs with different spacers in between the two imine bonds.
There is a report of electrochemical synthesis of gold nanoparticles (AuNPs) using poly[M(salen)] (M = Ni, Pd) by Tchepournaya et al.24 No report is available for synthesizing silver nanoparticles (AgNPs) with the help of salen or salen derivatives. In our present study, we have reported a novel photochemical strategy to synthesize both AuNPs and AgNPs with salen and salen-like molecules having different spacers between the two iminic bonds. The capping capability of different salen-like DSBs for both types of nanoparticles (NPs) is also reported here. It has also been shown that the salen and salen-like molecules possess a self-quenching fluorescence property which is reported here for the first time. In general, UV irradiation for more than 2 h causes excimer formation out of the quinone form of salen and salen-like molecules. We have shown that the fluorescence intensity of the substrate (salen and salenlike molecules) is further quenched in the presence of AuNPs which are photochemically generated in the reaction mixture. Fluorescence quenching by the fluorescence resonance energy transfer (FRET)25,26 mechanism is very common in the presence of metal NPs. On the contrary, AgNPs synthesized by a similar photoactivation reaction increases the fluorescence intensity dramatically. A great deal of research activity is now being directed for the fluorescence enhancement scheme with a variety of probe molecules considering the fluorophore to metal nanostructure proximity.27,28 The origin of signal enhancement is due to the interaction between the fluorescence probe and the surface plasmons of NPs. Owing to this, the radiative decay rate along with quantum yield are increased, resulting in fluorescence enhancement. The extent of enhancement lies in the geometry of the metallic nanostructures. The size and shape possess different surface plasmonic modes, causing drastic enhancement of florescence signals at the “hotspots”.29 Guo et al.30 demonstrated the importance of coupling between silver nanowires and an underlying silver film in fluorescence enhancement from a proximal molecule. Alteration of the thickness of the oxide separating the nanowire from the Ag film is responsible for a change in incident light polarization, resulting in
enhancement of fluorescence. Alteration of regions of high field above and between the nanowires becomes the main point of interest. A very interesting report was made by Lakowicz et al.31 that most of the self-quenching can be partially eliminated by proximity of the labeled protein to silver island films, suggesting the use of heavily labeled proteins and metallic colloids to obtain ultrabright reagents for use in immunoassays, imaging, and other applications. Scientists have proposed two mechanisms for fluorescence enhancement: (a) localized surface plasmon resonance (LSPR) at the surface of the metal NPs causing enhancement of the electromagnetic field and (b) the coupling between the surface plasmon field of the metal and the molecular dipole of probe molecules.32 The interaction of fluorophore and metal particle occurs through space, and maximum enhancement of fluorescence is observed when the mutual distance between them is about 70 100 Å. The quantum yield of the fluorophore and metalized surface governs the magnitude of enhancement. The density and shape of the metal particles are another point of attention. Shortened lifetime and often higher photostability associate with the brightness enhancement.33 Most of the works of fluorescence enhancement have been reported for a solid silver surface. There are rare examples of such phenomena in solution phase.34,35 Al-Kady et al.36 demonstrated chelation enhanced fluorescence for coumarin thiourea derivatives, where they took presynthesized AgNPs. In the present work, we have shown the surprising contrast of the fluorescence behavior of DSBs in solution in the presence of in situ generated AgNPs and AuNPs which may be helpful in various aspects of science.
’ EXPERIMENTAL SECTION Material and Instrument. All the reagents were of AR grade. Triple distilled water was used throughout the experiment. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), salicylaldehyde, and all the diamines were obtained from Sigma-Aldrich. NaOH was purchased from HiMedia Laboratories Pvt. Ltd. All glasswares were cleaned with freshly prepared aqua regia, 22139
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Scheme 1. Mechanism of Formation of Nanoparticles in the Presence of DSBs
subsequently rinsed with a copious amount of distilled water, and dried well before use. All the reagents were used without further purification. The sample solution was irradiated with a TUV 15W/G 15 T8 ultraviolet light (Philips India) source. All UV vis absorption spectra were recorded with a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India). FT-IR spectra were recorded in a FT-IR Nexus spectrophotometer (Thermo Nicolet). 1 H NMR spectra were obtained with a 400 MHz Bruker NMR instrument. X-ray photoelectron spectroscopy (XPS) analysis was carried out with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. The fluorescence measurement was carried out at room temperature using an LS55 fluorescence spectrometer (Perkin-Elmer, USA). TEM analysis was performed with an H-9000 NAR instrument (Hitachi) using an accelerating voltage of 300 kV.
’ METHOD OF PREPARATION Preparation of DSBs. Ethylenediamine (10
2
mol) in methanol was slowly mixed with methanolic salicylaldehyde (2 10 2 mol) solution with constant stirring. Then, the mixture was refluxed for ∼4 h. After cooling, a yellow precipitate was obtained. After filtration and washing 2 3 times with methanol, the yellow product was recrystallized from methanol.1 Melting point (127 °C), IR (Supporting Information, Figure S1), mass spectra (Supporting Information, Figure S2), and 1H NMR (Supporting Information, Figure S3) authenticate the synthesis of C1 (salen). Likewise, C2 (salprn), C3 (salben), C4, C5, and C6 were synthesized37 by using 1,3-propylenediamine, 1,4-butanediamine, o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine, respectively, in lieu of ethylenediamine. Figure 1 shows the structures of all six DSBs. Synthesis of AgNPs and AuNPs. A stock solution of 2.5 10 3 M DSB was prepared by dissolving an appropriate amount of DSB in 0.1 M aqueous NaOH solution. In a fluorescence cuvette, 3.2 mL of 0.1 M NaOH, 0.2 mL of DSB solution, and 0.1 mL of 10 2 M AgNO3 solution were mixed. The final concentration ratio of DSB to AgNO3 was maintained at 1:2. Then, the well-stoppered cuvette was irradiated under a 365 nm UV lamp for 3 h. The plasmon band for AgNPs at ∼400 nm remained masked within the absorption band of the Schiff bases (Supporting Information, Figure S4). Using a similar protocol, AuNP was produced while 0.1 mL of 10 2 M HAuCl4 was employed in lieu of AgNO3 and irradiation prolonged for 11 h. A red solution with a plasmon band maximum at 519 nm indicates the birth of AuNP when C2 is used. A similar procedure with C5 produced AuNPs with a plasmon band maximum at 530 nm. However, C1, C3, C4, and C6 could not produce stable gold hydrosol; rather, a precipitate of
flocculated gold particles deposited at the bottom of the cuvette upon photoirradiation.
’ RESULTS AND DISCUSSION The solubility of all the DSBs is very poor in distilled water. Hence, an alkaline aqueous solution is used to dissolve the DSBs. It has been shown by Selvakannan et al.38 that the ionization of the phenolic group of tyrosine at high pH transfers an electron to the silver ion, producing Ag(0) and resulting in the formation of quonone using thermal energy. Similarly, we presume that Au(0) and Ag(0) are produced from DSBs and the DSBs in turn are converted to the quinone forms. Scheme 1 describes the formation of Ag(0) from AgNO3 as a result of oxidation of DSB in solution. UV irradiation provides the required activation energy for the above process. Not only the 365 nm light source but also the 254 nm light source produces Ag(0) and Au(0) nanoparticles successfully. It is interesting to mention that simple heating or microwave irradiation causes flocculated NPs which are thrown at the bottom of the container. Visible light of wavelength 450, 550, and 650 nm is not at all efficient to reduce silver (evident from fluorescence studies) and gold salts in DSB solution (Supporting Information, Figure S5). UV light irradiation is essential in this system to obtain highly stable silver hydrosol with all the DSBs and gold hydrosol with C2 and C5 (Supporting Information, Figure S6). The stability of the photoproduced silver and gold hydrosol happens to be excellent. A noticeable decrease of the enhanced fluorescence intensity of the silver hydrosol is not observed even after keeping the solution undisturbed for a week. Again, no alteration in the extent of quenching of the exposed DSB after a week in the presence of AuNPs supports the excellent stability of the gold hydrosol in the context of fluorescence quenching. No solid mass was obtained by centrifuging both gold and silver hydrosol even at 10 000 rpm for 25 min, as they are sturdily capped by DSBs. Powder XRD is not the appropriate technique for determination of the oxidation state of gold and silver responsible for efficient quenching and drastic fluorescence enhancement, respectively, because the intense peaks of organic molecules (capping agent) conceal the metal. By boiling the solutions for 30 min, we obtained black masses from both of the sol systems. AgNPs are very much prone to react with aerial oxygen, resulting in the formation of Ag2O and AgO39 especially at the time of heating. The XPS information for the black masses may not bear true information regarding the oxidation state of the metals. We obtained XPS spectra of both the sol systems after freeze-drying (Figure 2). Peaks at 87.3 and 83.7 eV correspond to Au 4f5/2 and Au 4f7/2, respectively, as a proof of the zero oxidation state of gold.40 For the silver hydrosol, we found the peaks at 374.1 and 368.1 eV which can be assigned to Ag 3d3/2 and Ag 3d5/2, respectively, when silver is also in zero oxidation state.41 The zero oxidation number of photoproduced metal 22140
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Figure 2. XPS of gold and silver hydrosol after freeze-drying.
Figure 3. (a) TEM, (b) HRTEM, and (c) SAED images of AuNPs capped with C2 [C2:HAuCl4 = 1:2].
nanoparticles was further supported by HRTEM and SAED images. The HRTEM image of silver sol provides the lattice fringes: 0.236 nm, 0.250 nm which correspond to (111) and (1/3){422} reflections for an fcc lattice. A fringe spacing of 0.236 nm for the (111) plane was observed for gold sol also. The SAED image indicates sharp crystalinity, and the (111), (200), (220), and (311) planes are indexed from the SAED pattern. All of these guarantee the zero oxidation state of both of the photoproduced metal nanoparticles (Figures 3 and 4). Again, the characteristic lattice spacing between the (111) planes of Ag2O is reported to be 0.265 0.273 nm, which is not observed.42
The property of self-quenching in fluorescence, where the fluorophore and quencher are the same species, is observed for all the unexposed compounds (C1 C6) with no change in the wavelength of emission maxima with concentration. While the concentration is higher than ∼2 10 4 M for the compounds C1, C2, and C3, such quenching is observed. The self-quenching mechanism becomes operative for the compounds C4, C5, and C6, while their concentration is >5 10 5 M. Introduction of an aromatic ring in between two iminic bonds excels more pi-stacking which results in self-quenching even at a lower concentration range. Figure 5 shows the self-quenching property of 22141
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Figure 4. (a) TEM, (b) HRTEM, and (c) SAED images of AgNPs capped with C2 [C2:AgNO3 = 1:2].
Figure 5. Fluorescence study involving C2. Conditions for self-quenching >2.08 10 4 M and for normal fluorescence
