Condensation Product of 5-Bromosalicylaldehyde and Amino Phenol: Fluorescence Sensor for Ascorbic Acid and AND Logic Gate

Condensation product of 5-bromosalicylaldehyde and aminophenol (L) has been synthesised and characterised. Fluorescence of L enhances by 23 times on interaction with Ce 3+ while it is quenched completely by Ce 4+ . Ascorbic acid (AA) is a well known strong reducing agent and this property has been used to act L:Ce 4+ adduct as a uorescence “on” sensor for AA. AA reduces Ce 4+ into Ce 3+ and thereby increasing uorescence of L due to the formation of L:Ce 3+ adduct. Molecules which generally coexist with AA viz. Cholesterol, Glucose, Sucrose and Dopamine found not to interfere. The interaction of L with Ce 3+ , Ce 4+ and subsequently with AA has been veried with cyclic voltammetry.


Introduction
Ascorbic acid (AA), commonly known as vitamin c, exists both in plants and animals. It has strong antioxidant property and takes part in a number of physiochemical and biochemical processes in human body. There is report that AA is associated with many chronic diseases like -gout, skin disease, infertility, mucositis, cancer and HIV/AIDS [1][2][3]. Due to strong antioxidant property AA can stop change in taste and smell in foods and beverages and therefore used in food, beverages, medical formula, cosmetics etc. [4].
AA is not synthesised in human body and need to be su ciently supplemented through different food items in order to avoid many diseases. However excess intake of ascorbic acid may result health problems such as urinary stones, diarrhoea, stomach cramps etc. Hence determination of AA in different food items is of great importance.
There has been recent reports on uorescence sensing of ascorbic acid but most of them are based on nanoparticles and quantum dots (QD). Green emission uorescent based on silicon nanoparticle (SiNP) and bovine serum albumin templated MnO 2 nanosheets is reported. Initially the uorescence of SiNP is quenched by MnO 2 but AA on interaction reduces MnO 2 into Mn 2+ and SiNP gains back its uorescence [19]. Fluorescent "on" sensor for the detection of AA has been developed based on the uorescence resonance energy transfer (FRET) between graphene QDs and squaric acid (SQA)-Fe 3+ [20]. MnO 2 nanosheets were interacted with MoS 2 QD to quench its uorescence which is restored by interacting with AA due to reduction of MnO 2 [21]. Self assembly of 2,6-pyridine dicarboxylic acid sensitised Eu 3+ and carbon dots results resometric uorescence response towards AA [22]. The blue uorescence of SiNP was quenched by CoOOH nanoparticle and on interaction with AA the later nanoparticles are decomposed by redox process restoring the uorescence of SiNP and hence falicitating sensing of AA [24]. In another method the uorescence of CD is decreased by Ag NPs produced in situ by interaction of AA with Ag(I) in presence of Ag NP seeds [25]. Graphene QD was complexed with dopamine by electrostatic interaction and H-bonding which was then coordinated with Cu 2+ which quenched the uorescence of QDs and the uorescence quenching is removed by AA added to the solution resulting in AA sensing [26]. N doped CDs with high uorescence quantum yields have been reported which could detect AA by uorescence "off" mode due to inner lter effect [27]. In another reported method the uorescence of N doped CD is quenched by Cr(VI) rst, AA then reduces the Cr(VI) into Cr(III) restoring the uorescence [28]. CdTe QDs have been reported to show red shift in uorescence emission peak due to increase in its size on interaction with AA and hence acts as sensor for AA [29].
In this paper we report a very simple method for the detection of ascorbic acid where the uorescence of the probe is rst quenched by Ce 4+ and then ascorbic acid reduced it into Ce 3+ leading to very large increase in uorescence intensity. The method is found to be interference free from cholesterol, glucose, sucrose and dopamine. The probe acts as AND logic gate for uorescence output with Ce 4+ and AA as input. All the chemicals were either from Merck or Loba Chemie. The metal salts except Pb(NO 3 ) 2 , CaCl 2 and HgCl 2 were sulphates. Metal salt solutions were prepared in doubly distilled water obtained from quartz double distillation plant. The FT-IR spectra were recorded in a Perkin Elmer RXI spectrometer as KBr pellets, 1 H NMR and 13 C NMR spectra were recorded on a Bruker Ultra Shield 300 MHz spectrophotometer using DMSO D6 as solvent. The uorescence and UV/Visible spectra were recorded in HITACHI 2700 and Shimadzu UV 1800 spectrophotometer respectively using quartz cuvette (1 cm path length). Electrochemical experiments were performed at CHI (USA) electrochemical analyser work station 660D. Pt disc was used as working electrode, Ag-AgCl (3 N NaCl) as reference electrode and Pt wire as auxiliary electrode. Nitrogen gas was passed at slow rate through the solution to remove dissolved oxygen.

Experimental
Results And Discussion L in 1:1 (v/v) CH 3 OH:H 2 O (5×10 -4 M) shows uorescence spectrum on excitation with 270 nm photons in quartz cell of path length 1.0 cm. The emission was observed in 280 nm to 700 nm range with a maximum at 308 nm with intensity 360 (Fig. 1). Fig. 2 shows the uorescence spectra of L in presence of one equivalent of different metal ions. From the gure it is clear that Ce 3+ enhances uorescence of L signi cantly while Ce 4+ quenches the uorescence of L. Metal ions -Al 3+ , Li + , Na + , Pb 2+ , Cd 2+ , Hg 2+ , Mn 2+ , Mg 2+ , K + , Ca 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ do not effect uorescence spectra of L. Fig. 3 shows the uorescence spectrum of L in 1:1 (v/v) CH 3 OH:H 2 O at different added concentration of Ce 3+ . The l max value for L was found to shift from 308 nm to 350 nm in presence of Ce 3+ .   6 shows the effect of uorescence spectra of L in presence of one equivalent Ce 4+ and on subsequent addition of AA. It is observed that uorescence intensity increases with red shift in l max on addition of AA and nally the l max becomes 350 nm which is same to that of L in presence of one equivalent Ce 3+ . AA is a strong reducing agent and therefore it reduces Ce 4+ into Ce 3+ and the uorescence enhancement is observed. Fig. 7 shows the plot of uorescence intensity as a function of AA concentration which is linear. The effect of AA on L + Ce 4+ + AA has been examined by UV/Visible spectroscopy also. Fig. 9 shows the UV/Visible spectrum of L + Ce 4+ in presence of different added concentration of AA in 1:1 (v/v) CH 3 OH:H 2 O. In absence of AA peaks were observed at 284 nm and 422 nm for L + Ce 4+ in 1:1 (v/v) CH 3 OH:H 2 O. On addition of AA the peak at 422 nm does not undergo any change while the absorbance of the peak at 284 nm increases gradually with a shift in l max to 270 nm. Fig. 9, Inset shows the plot of absorbance versus AA concentration.
The interaction of L with Ce 4+ and L with Ce 4+ + AA has been veri ed by cyclic voltammetry at Pt working electrode using Ag-AgCl (3 M NaCl) in 1:1 (v/v) CH 3 OH:H 2 O (Fig. 10). The green curve is for L, the red curve is for L + Ce 4+ and the blue curve is for L + Ce 4+ + AA. The cyclic voltammograms for L + Ce 4+ is found to be quite different from that for L. Addition of AA leads to a cyclic voltammogram which is similar to that for L. This con rms that the interaction between Ce 4+ and L must be stronger than the interaction between Ce 3+ and L.
The fact that Ce 4+ quenches uorescence of L while Ce 3+ enhances it can be explained (Scheme 2) considering the ionic radii of Ce 4+ and Ce 3+ which are 115 pm and 101 pm respectively. Due to smaller size of Ce 4+ it can t into the hole created by ONO of L and bind to the two O of hydroxyl groups and the immine N of L. Due to its higher charge to size ratio Ce 4+ withdraw the electron density from the conjugation system of L strongly and therefore the uorescence is quenched. On the other hand Ce 3+ being bigger do not t into the hole and binds to the two O of the hydroxyls. Hence stops the PET process and uorescence enhances. This also explains the different nature of cyclic voltammograms of L in presence of Cr 3+ and Ce 4+ , since Cr 4+ affects the conjugation system of L, its presence greatly in uences the cyclic voltammogram of L while Cr 3+ does not affect the conjugation system of L and that is why the cyclic voltammogram is similar to that of L.

AND Logic Gate
The response of Ce 4+ and AA towards L with respect to uorescence forms the basis of AND Logic Gate truth    Fluorescence spectra of L in 1:1 CH3OH:H2O (v/v) at different added concentration of Ce3+, λex 270nm, λmax 348.6nm.      Change in the UV/Visible spectrum of L in 1:1 (v/v) CH3OH:H2O in presence of Ce4+ and different added concentration of Ascorbic acid.