The Preparation and Characterization of the Hexacyanides Immobilized in Chitosan

Mostramos neste trabalho alguns detalhes sobre a imobilização de complexos ferrocianeto e ferricianeto sobre a superfície do biopolímero quitosana. Os complexos imobilizados foram caracterizados por eletroquimica, espectrocopia de infravermelho e Mössbauer. Foi observado mudanças nos espectros de infravermelho e Mössbauer, e no voltamogramas cíclicos do ma te rial preparado com ferricianeto após exposição a luz do sol. Estas alterações indicam que o ferricianeto sofreu um processo de redução fotoinduzida na superfícies do biopolímero.

In re cent years in creased at ten tion has been given to sun light-induced pho to chem i cal ef fects.The photoreactivity of both or ganic and in or ganic iron complexes has been rec og nized for a long time, and many reports on the ef fects of light on the chem is try of iron have been pub lished over the last 100 years 1 .In this study the prep a ra tion and char ac ter iza tion of ferro cyanide and ferricyanide com plexes im mo bi lized in chitosan is de scribed.Chitosan is a polysaccharide usu ally pre pared by the N-desacetylation of chitin, through a ba sic hy dro ly sis re action, and largely con sists of glucosamine units and a smaller per cent age of N-acetyl-glucosamine res i dues (Fig. 1) 2 .Chitosan has a sim i lar struc ture to that of cel lu lose, but shows many prop er ties that dif fer from the lat ter, for ex ample, chitosan is con sid ered more chem i cally ver sa tile than cel lu lose, due to the pres ence of an amino group.This group gives chitosan sol u bil ity in or ganic ac ids (ace tic acid, for mic acid) and di luted in or ganic ac ids (chloridric acid).This poly mer has been the ob ject of con tin ued study for sev eral de cades.In par tic u lar, its metal-binding property has led to the use of chitosan as a low-cost ma te rial in, waste wa ter treat ment plants and streams con tam i nated by the ac cu mu la tion of toxic met als re leased by in dus trial activ i ties [3][4][5] .
In a re cent re port chitosan was used in the im mo bi li zation and char ac ter iza tion of ni trite, per manga nate, io dide Ar ti cle and ferri cyanide by strip ping voltammetry with a mod i fied car bon paste elec trode (MCPE).The im mo bi li za tion in the MCPE oc curs through the elec tro static in ter ac tion be tween the NH 3 + groups in the protonated chitosan (pH be low 3.5) and the re spec tive an ions 6 .
The chem i cally mod i fied elec trode (CME) for electroanalysis has in creas ingly been the sub ject of research.The elec trodes are made by in cor po rat ing spe cific chem i cals and group ing them on the elec trode sur face.The use of re dox poly mers as elec trode mod i fi ers to in duce electrocatalysis has drawn con sid er able at ten tion.Such poly mers can pro vide a sup port for the site that me di ates the ex change of elec trons be tween the elec trode and the solu tion 7 .
The pur pose of the pres ent study is to pre pare and charac ter ize the ferri cyanide and ferrocianyde ad sorbed on the chitosan sur face.This ma te rial was used in the prep a ra tion of the CME in an electrocatalysis ex per i ment.
In the course of the char ac ter iza tion of the poly mer redox, it was ob served that the ion ferri cyanide which was adsorbed on a chitosan sur face un der went a photoinductive re duc tion pro cess.

Ex per i men tal
An a lyt i cal grade po tas sium ferri cyanide and ferrocyanide from Merck were used.Chitosan (20% N-acetylated) was pre pared by the ba sic hy dro ly sis of chitin, fol low ing the method de scribed in the lit er a ture 8,9 .The im mo bi li za tion of the ferri cyanide com plex on a chitosan sur face was done as fol lows: 0.6 g chitosan was im mersed in a 0.1 M ferri cyanide so lu tion (50 mL) and the pH of this so lu tion was ad justed to 3 us ing HCl.This so lution was stirred for two hours and pro tected from light.The solid was sub se quently fil tered and exhaustively washed with dis tilled wa ter to re move the ex cess ferri cyanide, and was then dried un der vac uum at room tem per a ture.The immo bi li za tion of ferro cyanide was done in a sim i lar way.The cy clic voltammograms were ob tained us ing a cy clic voltammeter model CV-27 (Bioanalytic Sys tem, Inc.) connected to an X-Y Omnigraphic 100 reg is ter (Hous ton Instru ments ).A three-electrode sys tem was used, with a car bon paste elec trode as the work ing elec trode, a (Ag/AgCl -KCl sat.) ref er ence elec trode and a plat i num wire as the aux il iary elec trode.The car bon paste mod i fied elec trode (CPME) was pre pared by mix ing 0.4 g of elec trolytic car bon pow der (Rekipro) with 0.2 g of min eral oil (Nujol) and 0.06 g of poly mer.The paste was packed into glass tube, which gave a geo met ric sur face area of ap prox imately 0.07 cm 2 .The in fra red spec tra were ob tained in KBr disk on a IR-FT Bomen MB-100 spectrophotometer.
The Mössbauer spec tra were ob tained at room tem per ature, us ing 24 mCi 57 Co source in a ma trix of Rh, and detected by a Kr-CO 2 pro por tional coun ter (Reuter Stokes) cou pled to mul ti chan nel an a lyzer (EG&G ORTECH).The iso mer shifts were cal i brated with α-iron foils and ref erenced against so dium nitroprusside (line wide at one-half the max i mum of 0.25 mm s -1 .
The Mössbauer spec tra were ad justed by Lorentzian curves uti liz ing the "Normos" ver sion PC 10 , and de composed us ing the Grapher pro gram 11 .The mea sure ments for the Mössbauer spec tros copy were ob tained from solid sam ples of the ferri cyanide ion com plex im mo bi lized on a chitosan sur face ir ra di ated with sun light, and com pared with free ferro cyanide and ferri cyanide.

Re sults and Dis cus sion
The pres ence of ferri cyanide ad sorbed on a chitosan surface can be ob served by the green ish yel low col or ation charac ter is tic of this an ion.This color grad u ally changes to a blu ish shade when sub mit ted to sun light ra di a tion.The same blu ish shade is ob served in the poly mer sam ple pre pared with ferro cyanide.The quan tity of ferri cyanide ad sorbed on the poly mer sur face de ter mined by the flu o res cence of the X-ray was 2.74 x 10 -3 mol g -1 , and for the ferro cyanide it was 2.6 x 10 -3 mol g -1 , de ter mined by atomic ab sorp tion.
The cy clic voltammogram of the freshly pre pared ferricyanide com plex ad sorbed on the chitosan sur face and that of the free ferri cyanide com plex are show in Fig. 2. The corre spond ing and elec tro chem i cal pa ram e ters are shown in Ta ble 1.The half-wave po ten tial, E 1/2 , for the cou pled Fe(II/III) of the ferro cyanide ad sorbed on the poly mer is 245 mV, and the po ten tial for the free ferro cyanide is 205 mV, both mea sured against the ref er ence elec trode.The in ter action be tween the cyanides of the com plex and the hy drox yls of chitosan by hy dro gen bond ing re duced the δ do nor abil ity of the cyanides, in creas ing the ef fec tive nu clear charge of the me tal lic cen ter, shift ing the ox i da tion po ten tial to more pos i tive val ues.
Fig ure 3A shows the de pend ence of the cur rent on the inten sity with the scan rate.The ra tio be tween the an odic (i pa ) and the cath odic (i pc ) peak cur rents is near unity (Ta ble 1) , char ac ter is tic of a re vers ible sys tem.Lin ear ity of the an odic cur rent ver sus the square root of the scan rate (Fig. 3B, plot I pa vs. v 1/2 ) was also ob served, with a cor re la tion co ef fi cient of 0.994, the lin ear ity of these plots in di cat ing the dif fu sion lim i ta tion of the cur rents within the range of the scan rates in ves ti gated 13 .The great dif fer ence be tween the an odic and catodic peak po ten tials, ∆E p , is at trib uted to the IR drop in the ferri cyanide im mo bi lized on the poly meric ma trix 13,12 .The slight dis place ment of E 1/2 due to its scan ning rate shows that the ki netic of the trans fer pro cess is quite fast in this case 14 .
Fig ure 4 shows the voltammogram of the ferro cyanide and ferri cyanide com plex ad sorbed on the chitosan sur face, with the elec tro chem i cal pa ram e ters shown in Ta ble 1. Figure 5 shows a voltammogram of ferri cyanide ad sorbed on the chitosan sur face af ter ex po sure to sun light.The broad ening of the ox i da tion and re duc tion peaks, as well as the displace ment of the E 1/2 value in di cate the pres ence of more than one spe cies on the poly mer sur face.The voltammogram of the ferro cyanide spe cies ad sorbed on the chitosan does not un dergo any change when sub mit ted to sun light ra di a tion.
The in fra red spec trum of ferri cyanide ad sorbed on the poly mer shows a band at 2114 cm -1 , at trib uted to the CN stretch ing fre quency 15 (Fig. 6A).This band is in the same region as that of the free ferri cyanide, in di cat ing that the adsorp tion of the com plex with the chitosan sur face is of an elec tro static na ture.A stron ger in ter ac tion would dis place the CN stretch ing band to higher en ergy re gions 16,17 .When the poly mer was sub mit ted to sun light, the ap pear ance of a new band at 2039 cm -1 was ob served, which has been at tributed to the ferro cyanide spe cies 18 .The ap pear ance of this new band is ac com pa nied by the dis ap pear ance of the band at 2114 cm -1 (Fig. 6B), in di cat ing that the photoinductive reduc tion pro cess is op er at ing.The in fra red spec trum of the ferro cyanide ad sorbed on the poly mer shows a band at 2035 cm -1 (Fig. 6C), sit u ated in the same re gion as that of free ferro cyanide 13 .
In or der to con firm the pres ence of the spe cies found on the poly mer sur face us ing the tech nique cited above, Mössbauer spec tros copy was also used.Fig ure 7A shows the Gaussi an deconvoluted Mössbauer peaks for the polymer pre pared with ferri cyanide and sub mit ted to sun light radi a tion.In this fig ure, the most in tense peak shows an iso mer shift ( δ) of -0.03 mm s -1 , at trib uted to the ferro cyanide species ad sorbed on the poly mer (Fig ure 7A(b)).The mea sured δ are in good agree ment with of the free ferro cyanide spe cies (-0.04 mm s -1 ), as show in Fig. 7B.
Fig ure 7A(a) also shows that the ferri cyanide spe cies adsorbed on the poly mer sur face, with δ of -0.07 mm s -1 and a split ting ∆ of 0.36 mm s -1 , are in good agree ment with the free ferri cyanide spe cies, ( δ of -0.11 mm s -1 and a ∆ of 0.28 mm s -1 ), Fig. 7C.
Fig ure 7A(c) shows the pres ence of a third spe cies with (δ) of 0.06 mm s -1 and a ( ∆) of 0.80 mm s -1 , which can be attrib uted to the spe cies [Fe(CN) 5H2O] 3-ad sorbed on chitosan 18,19 .These val ues are very close to those for the same free spe cies, ( δ) 0.06 mm s -1 and ( ∆) 0.79 mm s -1 .These data con firm the fact that to tal con ver sion of ferricyanide to ferro cyanide does not oc cur, ac cord ing to the results showed by the in fra red spec tra.
A pos si ble de com po si tion of the re dox poly mer for the for ma tion of the Prus sian and Turnbull blue spe cies on the poly mer sur face are dis carded when com pared with the data from the Mössbauer spec tros copy and elec tro chem i cal data in the lit er a ture 13,20 .The re duc tion of ferri cyanide to ferro cyanide ocurrs only when the sam ple is ex posed to sun light.The re duc tion of ferri cyanide com plexes to ferro cyanide was also observed by Clearfied and co-workers in zir co nium polyimine phosphonates 21 .The photo in duced re duc tion pro cess of the ferri cyanide com plex on the chitosan sur face can be explained by the mech a nism of en ergy trans fer in the poly mer.This means that it oc curs be tween the do nor and re ceiver elec tron spe cies in dif fer ent mol e cules (intermolecular inter ac tions) or be tween do nor and ac cep tor groups in the same poly meric chain (intramolecular in ter ac tions).Assuming that when chitosan is sub mit ted to sun light ra di a tion, aminic ni tro gen ad sorbs a pho ton and is pro moted into an excited state, the elec tron is later trans ferred to ferri cyanide through a mech a nism of intramolecular trans fer.Since the poly mer is flex i ble, col li sions may oc cur be tween the do nor groups (ni tro gens) and the ac cep tor (ferricyanides) of the same poly meric chain, re duc ing ferri cyanide to ferrocyanide 22 .The photoinductive re duc tion mech a nism is a remark able con tro versy in the lit er a ture 1,19,23 , al though it is of fun da men tal im por tance to un der stand it.Many re ac tions on the poly mer sur faces have im por tant im pli ca tions: these pro cesses are im por tant in ex plain ing the de te ri o ra tion of ma te ri als in cat a lytic in dus trial pro cesses and the phe nomena of pho to graphic im ages.

Fig ure 1 .
Fig ure1.The chitosan struc ture4 .The polysaccharide was pre pared by the N-desacetylation of chitin.

Fig ure 6 .
Fig ure6.The in fra red spec tra.(A) ferri cyanide ad sorbed on the recently pre pared poly mer sur face sam ple, (B) ir ra di ated sam ple, and (C) ferro cyanide on the chitosan sur face.

Fig ure 4 .Fig ure 5 .
Fig ure4.Cy clic voltammograms of (1) the ferri cyanide spe cies, and (2) ferri cyanide, on the chitosan sur face af ter ex po sure to sun light radi a tion, in KCl 0.1 M, at pH 3.0, and at a scan rate of 100 mV s -1 .