Mn-Based Coordination Polymer:Facile Synthesis,Structure and Application in Glucose Electrochemical Sensing

ZHAO Lu CHEN Tong⁃Dan WANG Han⁃Ye LI Chen⁃XiLI Jiang

(School of Materials Science and Engineering,Chang'an University,Xi'an 710064,China)

Abstract:Utilizing the rigid 6⁃(3⁃pyridyl)isophthalic acid(H2PIAD)linker,one Mn（Ⅱ）⁃based coordination polymer{[Mn(PIAD)(DMF)]·H2O}n(1)was prepared firstly.In order to improve the electrocatalytic activity of glucose sens⁃ing,a composite material(Ag@1)was prepared by the strategy of post synthesis of Ag nanoparticles(NPs).The elec⁃trocatalytic performance of the glassy carbon electrode(GCE)modified by Ag@1 was evaluated by chronoamperome⁃try method at the optimized application potential,and coordination polymer 1 provided a fixed substrate for the uni⁃form distribution of Ag NPs on its surface.Ag@1 sensor can maximize the electrocatalytic synergistic effect of the combination of Ag and 1 on glucose oxidation.The results reveal that modified GCE by Ag@1 had good performance for the detection of glucose with low detection limit(6.36 μmol·L-1),good selectivity and sensitivity(166.71 μA·L·mmol-1·cm-2).CCDC:2080639.

Keywords:manganese;coordination polymer;Ag nanoparticles;glucose sensor

0 Introduction

Diabetes mellitus is deemed a complex,chronic illness and have become one of the biggest threats to human health[1].Therefore,it is urgent to accurately monitor glucose levels for reducing the risk of its long⁃term complications.At present,much more ongoing and reliable glucose sensors are very important.The electrochemical biosensors depend mainly on the activ⁃ity of enzymes,such as glucose oxidase(GODx)[2⁃3].However,due to its complex preparation,the activity of biosensors can be easily affected by temperature,pH value,humidity,and toxic chemicals[4⁃7].Also,most enzyme⁃based sensors involve complex immobilization steps of glucose oxidase,which increases the uncertain⁃ty of bio⁃sensing[8⁃9].Accordingly,non⁃enzymatic elec⁃trochemical sensors can be profitable in low⁃cost,struc⁃tural simplicity and long⁃term stability[10⁃11].

To date,coordination polymers(CPs),as an impor⁃tant type of nanomaterial,have received considerable attention under the context of electrochemistry recent⁃ly[12],such as electrochemical sensing[13⁃16],electrocatal⁃ysis[17⁃18],bio ⁃imaging[19],supercapacitors[20⁃22],and fuel cell catalyst[23].Thanks to the advantages of CPs,such as high specific surface area,tunable structure,rich active sites and tailorable structure[24],these prospec⁃tive features stimulate that CPs as a carrier for the immobilization of specific functional nanomaterials.Although the application success of CPs in the non⁃enzymatic sensors[25],CPs with poor conductivity can⁃not play a significant role in avoiding inherent defects when applied to electrochemical sensing[26].Therefore,it is worth to consideration combining other functional nanomaterials with CPs to form a hybrid material achieving highly efficient sensing.Metal(e.g.,Pt,Au,Ag)nanoparticles(NPs)could increase specific surface areas and enhance mass transport ability in electro⁃chemical sensors.And the NPs are usually combined with other materials such as porous carbon[27⁃29],metal organic frameworks[30⁃32]and graphene[33⁃36]used for electrochemical sensing to enhance their catalytic activity.Chang et al.[37]developed a facile and clean method by using ascorbic acid as a mild reductant to synthesize Pt/graphene composites under the assis⁃tance of polyvinyl⁃pyrrolidone(PVP).Chen et al.[38]synthesized Ni(SA)2(H2O)4CP and transformed it into porous NiO nanorods with excellent electrocatalytic properties.The porous NiO modified glassy carbon(p⁃NiO/GC)electrode showed a wide linear range(0.01⁃5 mmol·L-1),which could be used for determination of glucose in human serum samples.Among various metal NPs,Ag NPs possess high conductivity and biocompati⁃bility,which increases their demand in the field of sensing[39].However,NPs usually have the phenomenon of agglomeration,decreasing the active site[40].In this investigation,Mn（Ⅱ）⁃based CPs was introduced to pro⁃vide the matrix for the uniform distribution of Ag NPs.Thus,Ag NPs can be considered as the metal of choice together with CPs to amplify the electrocatalytic activi⁃ty for glucose oxidation.

Herein,employing ligand 6⁃(3⁃pyridyl)isophthalic acid(H2PIAD),one novel CPs,namely{[Mn(PIAD)(DMF)]·H2O}n(1),was constructed from the ligand and Mn（Ⅱ） ions in specific solvent systems.Notably,com⁃pound 1 was chosen as a substrate,and Ag NPs were deposited onto the surface of 1.The obtained Ag@1 was further used to modify glassy carbon electrode(GCE)to establish an electrochemical sensing platform for glucose detection.The modified Ag@1/GCE pos⁃sessed a suitable sensing performance for the sensitivi⁃ty,selectivity,good stability and low detection limit(6.36 μmol·L-1).

1 Experimental

1.1 Preparation of{[Mn(PIAD)(DMF)]·H2O}n(1)

A mixture containing MnCl2·4H2O(0.013 5 g,0.068 mmol),6⁃(3⁃pyridyl)isophthalic acid(0.019 5 g,0.08 mmol),deionized water(1.0 mL),N,N⁃dimethylfor⁃mamide(DMF,3.0 mL)was ultrasonic mixing for 10 min,and transferred to autoclave and heated at 105℃for three days.After cooling,faint yellow crystals were collected by filtration and dried at 40℃for 4 h.Yield:68%based on Mn.Anal.Calcd.for C16H16MnN2O6(%):C,49.62;H,4.16;N,7.23.Found(%):C,50.53;H,4.24;N,7.18.FT⁃IR(KBr,cm-1):3 414(w),3 069(w),1 580(s),1 388(s),1 171(m),1 097(w),1 036(m),1 012(m),924(m),860(m),821(s),783(s),763(m),735(m),706(s),688(s),659(m),647(m),621(m),542(m),474(m).

1.2 Preparation of Ag@1

Ag@1 was synthesized based on a modified report⁃ed method[41].Compound 1(50 mg)was added to 10 mL H2O/DMF solution(1∶3,V/V)containing AgNO3(1 mmol·L-1)and stirred overnight.Then the collected suspension was dispersed in 10 mL H2O/DMF solvent(1∶3,V/V),followed by rapidly adding 5 mL of the same mixed solvents containing NaBH4(0.1 mmol).The obtained sample was centrifuged and washed with ethanol five times.

1.3 Preparation of Ag@1 modified electrode

The glucose sensors as⁃fabricated by Mn（Ⅱ）⁃based CPs(1)was assembled on the surface of the electrode to form a Nafion/Mn（Ⅱ）⁃based CPs sensor film.Before modifying the GCE substrate,the GCE was treated with 0.05 μm alumina slurry and rinsed by ultrasonic clean⁃ing with ethanol and deionized water for 5 min,respec⁃tively.Then,1 mg sample of 1 was dispersed into 1 mL ethanol and treated by ultrasonication for 30 min to form a homogeneous suspension.Each time before elec⁃trode was modified by suspension,ultrasonic treatment for another 30 min was needed.Afterwards,the suspen⁃sion(5 μL)and Nafion solution(2 μL,1%)were suc⁃cessively dripping on the GCE surface based on elec⁃trostatic interactions to obtain the Mn（Ⅱ）⁃based CPs modified electrode(1/GCE).Through a similar method,Ag@1⁃modified electrode(Ag@1/GCE)was obtained,which was dried at room temperature before further measurement.

1.4 Characterization and electrochemical test

Scanning electron microscopy(SEM,Hitachi S⁃4800,Japan)was used to observe the hybrid morpholo⁃gy at the voltage of 2 kV.IR spectra were acquired as KBr discs using a Bruker EQUINOX⁃55 FT⁃IR spec⁃trometer varying from 400 to 4 000 cm-1.X⁃ray photo⁃electron spectra(XPS,ESCALAB 250xi)and powder X⁃ray diffraction measurements(PXRD,Bruker D8 ADVANCE)were used to observe the chemical compo⁃sition and structure of the obtained sample.X⁃ray diffractometer employing CuKαradiation(λ=0.154 18 nm)within a 2θrange of 5°⁃50°.Moreover,the operat⁃ing voltage and current are 40 kV and 40 mA,respec⁃tively.Thermogravimetric analyses(TGA)curves were collected with a NETZSCH STA 449 CTG/DTA equip⁃ment under N2stream at a heating rate of 10℃·min-1.Electrochemical measurements were conducted on the electrochemical workstation(IVIUM Vertax)with a three⁃electrode configuration containing a working electrode of a modified GCE(Φ=3 mm),a reference electrode of Ag/AgCl(3 mol·L-1KCl)and a counter electrode of platinum wire(0.5 mm×37 mm).

All the cyclic voltammetry(CV)curves were col⁃lected in 0.1 mol·L-1NaOH aqueous electrolyte with and without glucose(1 mmol·L-1).Preliminary to elec⁃trochemical test,Ag@1/GCE was scanned ten times in electrolyte containing 0.1 mol·L-1NaOH until CV curves coincided,thus activating Ag@1/GCE.The chronoamperometric responsesofAg@1 electrode towards glucose were measured at the applied voltage of 0.85 V(vs Ag/AgCl),then the stability and repeat⁃ability test were carried out.During the whole process of chronoamperometric test,the electrolyte was contin⁃uously stirred by magneton and glucose was added at the same time interval of 50 s.

1.5 Crystallographic data collection and refinement

Crystallographic data for 1 was collected by using aBrukerSMARTAPEXⅡCCDdiffractometer equipped with graphite monochromated MoKαradia⁃tion(λ=0.071 07 nm).The structure was solved by direct methods and refined onF2by full⁃matrix least⁃squares procedures with SHELXL program.The non⁃hydrogen atoms were refined anisotropically,while the hydrogen atoms were added to their geometrically ideal positions and refined isotropically.The crystallograph⁃ic data and structural refinement parameters are given in Table 1,and selected bond lengths and angles are listed in Table S1(Supporting information).

Table 1 Crystallographic data and structural refinement parameters of 1

CCDC:2080639.

2 Results and discussion

2.1 Structural description

Single crystal X ⁃ray diffraction analysis demon⁃strates that complex 1 is a 2D framework.Complex 1 belongs to the tetragonalP21/nspace group.As shown in Fig.1a,the asymmetric unit consists of one Mn（Ⅱ）center,one PIAD2-ligand,one DMF molecule and one free water molecule.Mn1 with a distorted octahedral coordination geometry is linked to one N atom and four O atoms from four PIAD2-ligands,and one O atom from one DMF molecule.The Mn—O bond distances vary from 0.210 0(2)to 0.235 1(2)nm,and the Mn—N bond distance is 0.228 3(3)nm.The O—Mn—Oangles vary from 56.95(8)°to 151.30(9)°,and the N—Mn—O angles vary from 83.15(9)°to 177.87(10)°.

Fig.1 (a)Coordination environment of Mn（Ⅱ）ion in 1;(b)2D layered network of 1 viewed along a axis;(c)2D structure of 1 viewed along b axis;(d)4,4⁃connected topological net of 1,where pink nodes and blue stick represent Mn（Ⅱ）ions and ligands,respectively

The carboxylate groups in PIAD2-ligand adopt two coordination fashion:(κ1⁃κ1)⁃μ1⁃COO-and(κ1⁃κ1)⁃μ2⁃COO-,to link Mn（Ⅱ） ions and further extend to a 2D sheet(Fig.1b and 1c).The neighboring 2D sheets are held together to construct a supramolecular 3D struc⁃ture.Topologically,both Mn1 and PIAD2-ligands can be simplified as 4⁃connected nodes.The structure of 1 represents(4,4)⁃connectedsqltopological net with a point symbol of(44·62)(Fig.1d).

2.2 Characterizations

In this work,powder⁃like 1 was dispersed in the solution containing Ag+precursors and then reduced by NaBH4to yield Ag@1 nanocomposite.As⁃prepared Ag@1 was also verified by IR,TGA,SEM,PXRD anal⁃ysis.The SEM coupled with energy⁃dispersive X⁃ray spectroscopy(EDX)was used to further visualize the coverage of Ag NPs on modified matrix of 1.As shown in Fig.2a,most of 1 had a blocky shape with a size ofca.200 μm along the length.It is explained that 1,as a matrix,provides a large surface for carrying more Ag to enhance Ag@1 electrocatalytic activity.When Ag NPs were embedded onto 1,as shown in Fig.2b,Ag@1 nanocomposite with a slightly smaller particle diameter kept the original morphology.It could be seen within the measured area from Fig.2c that Ag@1 comprised of the elements of C,N,O,Mn and Ag.The XPS spectra of Ag@1 further confirmed the presence of these ele⁃ments(Fig.3).The Ag3dpeaks appeared at 368.14 and 374.24 eV,further suggesting that Ag has been suc⁃cessfully loaded on the surface of 1.Moreover,two Ag peaks had a slight shift to the left,respectively,which could be assigned to the metallic Ag NPs[42].Besides,the structural stability of Ag@1 was measured by TGA(Fig.S1).For Ag@1,the similar but not identical TGA curve also indicated the intact frameworks structure of 1.It should be noted that the weight of the final prod⁃uct for Ag@1 was higher than 1,which was consistent with Ag NPs in composites.The IR spectrum of 1 is depicted in Fig.S2.

Fig.2 Morphology characterization and element analysis of Ag@1:SEM images of(a)1 and(b)Ag@1;(c)EDX spectrum of Ag@1;(d)Relative composition map images of Mn,Ag and C elements

Fig.3 XPS spectra of(a)Ag@1 and(b)Ag3d in Ag@1

From the PXRD pattern of 1(Fig.S3),all the peaks matched well with the simulated pattern of 1,confirming the phase purity of 1.For Ag@1,the diffrac⁃tion peaks of 1 could be well identified,revealing its retained original structure after embedding Ag NPs onto 1.In addition,after 1 was immersed in 0.1 mol·L-1NaOH for three days,the PXRD pattern showed that the structure of 1 could keep stable in 0.1 mol·L-1NaOH.Both TGA and PXRD results corroborate suc⁃cessful integration hybrids of Ag NPs with 1(Fig.S1 and Fig.S3).This gives enough evidence illustrating that Ag@1 can be designed and constructed as a novel non⁃enzymatic glucose sensor.

2.3 Electrochemical sensing

Since 1 and OH-species are involved in the elec⁃tro⁃oxidation of the glucose,it is expected that OH-plays a role in the catalyst process.Previous studies ex⁃plored the effect of NaOH concentration on the glucose oxidation current in the literature[43].Hence,0.1 mol·L-1NaOH electrolyte was used for further electrochemi⁃cal test of glucose oxidation efficiently.

The CV curves of GCE,1/GCE and Ag@1/GCE were investigated in the absence and presence of 1 mmol·L-1glucose with the whole potential window from-1.0 to 1.0 V.As shown in the inset of Fig.4a,the CV curve of bare GCE did not appear as a redox peak.Meanwhile,it was found that the presence or absence of glucose did not alter the curve trails,revealing the bare GCE electrode had negligible catalytic activity for glucose oxidation.However,after the redox⁃active 1 was coated on the GCE,a small reduction current was received.With the addition of glucose toward 1/GCE,the visible increase of redox peaks was observed at peak Ⅲ/Ⅳ,which demonstrate the moderate electro⁃catalytic performance of 1 for glucose oxidation.For 1,two pairs of asymmetric redox peaks position corre⁃sponding to the peak Ⅲ/Ⅳ in the inset of Fig.4a con⁃firm redox activity,and could be found at around 0.41 and-0.61 V,respectively.

Fig.4 (a)CV curves(Scan rate:50 mV·s-1)of the response with and without 1 mmol·L-1glucose in 0.10 mol·L-1NaOH solution for Ag@1/GCE and GCE,1/GCE(Inset);(b)CV curves(Scan rate:50 mV·s-1)of Ag@1 electrode in 0.10 mol·L-1NaOH solution containing 1⁃5 mmol·L-1glucose(Inset:enlarged view of 0⁃0.6 V range);(c)CV curves of Ag@1 electrode at different scan rates(25,50,75,100 and 125 mV·s-1)in 0.10 mol·L-1NaOH solution;(d)Plots of the square root of scan rate vs peak current at anodic and cathode for Ag@1/GCE

As shown in Fig.4a,upon adding 1 mmol·L-1glu⁃cose,for peakⅠ,Ag@1/GCE exhibited a much higher anodic current in the presence of glucose than that in the absence of glucose.The highest anode peak current showed that the introduction of Ag NPs significantly improved catalytic activity of 1 as catalyst,which guar⁃anteed that Ag@1/GCE had better performance as a glucose sensor.And the possible redox reactions for glucose on the surface of Ag@1/GCE could be assigned to the reversible transition in the oxidation state between Mn（Ⅱ） and Mn（Ⅳ） ions[44⁃46].First,the Mn（Ⅱ）center would be oxidized to Mn（Ⅳ）.Then the oxidative Mn could catalyze glucose oxidation to generate gluco⁃nolactone[47⁃48].

Besides,the distinctive feature of CV curves between 1/GCE and Ag@1/GCE appeared in a few aspects:the position of current start rising for Ag@1/GCE shifted to around 0.05 V(vs Ag/AgCl);the poten⁃tials of the anodic peak for Ag@1/GCE centered at 0.34 V(vs Ag/AgCl),lower than 1/GCE electrode at 0.41 V;the current responses for both 1/GCE and Ag@1/GCE gave a soared current signal to ensure both the electrodes can catalyze the electrochemical oxidation of glucose.The catalytic mechanism process of Ag@1/GCE is shown in Fig.5.

Fig.5 Catalytic mechanism process of Ag@1/GCE for glucose oxidation

To further investigate the ability of Ag@1 to cata⁃lyze and oxidize glucose under alkaline conditions,the CV curves of Ag@1 in the presence of glucose with different concentrations(1.0⁃5.0 mmol·L-1)were tested subsequently with an electrolyte solution containing 0.1 mol·L-1NaOH(Fig.4b).With the increase of the glucose concentration,an increasing trend can be observed in the anode peak （Ⅱ） current(up to 200 μA).Meanwhile,the oxidation peak potential shifted slightly positively with the increasing glucose concentration,indicating that Ag@1 has excellent catalytic oxidation ability for glucose.

Furthermore,the CV curves at various scan rates were collected for Ag@1/GCE(Fig.4c).Notably,as scan rate increased from 25 to 125 mV·s-1,the poten⁃tials of anodic peaks（Ⅰ）and cathodic peaks（Ⅱ）become higher and lower,respectively,which can be related to the electrochemical relaxation phenomenon.Besides,as shown in Fig.4d,the currents at anodic(Ipa,μA)and cathodic(Ipc,μA)peaks for Ag@1/GCE were directly proportional with the square root of the scan rate(v1/2),whichprovesoutthattheelectrochemicalprocessofAg@1 toward glucose oxidation is diffusion⁃controlled[49].

2.4 Optimization for sensor ability of Ag@1

Working potential is an indispensable factor for optimization performance of a sensor.Through continu⁃ous injection of 1.0 mmol·L-1glucose,the current response under three voltages of 0.8,0.85 and 0.9 V was studied,so as to optimize the sensing ability of Ag@1.As depicted in Fig.S4,the three applied poten⁃tials exhibited similar current response trend.Mean⁃while,the current signal for 0.8 V was so weak that it increased the error in the quantitative calculation of glucose concentration.The potential at 0.9 V was not conducive to the sensitivity of Ag@1 sensor.Therefore,the potential of 0.85 V had relatively less influence on the currents compared with 0.9 V,and a potential of 0.85 V was used for theI⁃tcurve test.As shown in Fig.6a,the consecutive step changes after each glucose injection inI⁃tplot revealed that the glucose response signal was positively correlated with glucose concentra⁃tion and the current signal reached its steady⁃state value(95%of the maximum)within 6 s.The calibra⁃tion curve(Fig.6b)showed the linear relationship be⁃tween the current signal(I,μA)and glucose concentra⁃tion(c,mmol·L-1)in a range of 10 ⁃4 540 μmol·L-1,which can be expressed by a linear equation:I=11.67c+5.21(R2=0.99).The sensitivity was calculated to be 166.71 μA·L·mmol-1·cm-2,and a low detection limit(LOD)of 6.36 μmol·L-1was found based on the signal⁃to⁃noise ratio of 3 for Ag@1/GCE sensor.A comparison of the analytical performance for this Mn（Ⅱ）⁃based CPs with other non⁃enzymatic glucose sensors is shown in Table 2.In this work,the combination of Ag NPs and CPs to form Ag@1 composites can reduce the oxidation potential of glucose oxidation and improved its catalyt⁃ic ability.Ag@1 sensor was offered to have a wide linear dynamic range and high sensitivity for glucose detection.It is conjectured that Ag@1/GCE well⁃performance arises from the following reasons:（ⅰ）the Mn（Ⅱ）⁃based CPs as supporting structure offers a plat⁃form;（ⅱ）the excellent electronic conductivity of Ag NPs and Nafion film could provide reliable interaction towards the glucose molecules;（ⅲ）the presence of metal active sites of Mn⁃CPs enables large amounts of glucose molecules on the surface of Ag@1/GCE for efficient oxidation.

Fig.6 (a)I⁃t curve of Ag@1/GCE by continuously addition of glucose in 0.1 mol·L-1NaOH electrolyte at an applied potential of 0.85 V(vs Ag/AgCl);(b)Calibration curve between glucose concentration and its current signal for Ag@1/GCE at 0.85 V(vs Ag/AgCl)

Table 2 Comparison of electrocatalytic abilities between Ag@1/GCE and other non-enzymatic sensors

2.5 Selectivity,stability and repeatability of Ag@1 sensor

The anti⁃interference performance is also an essential condition for a glucose sensor.The specificity of Ag@1 sensor was investigated by the chronoam⁃perometry response to the interferences.The interfer⁃ing substances,such as uric acid(UA),ascorbic acid(AA),sucrose,lactose,galactose,fructose,usually coexist with glucose inevitably in human serum,and their concentrations are less than 30 times that of glucose.Fig.7a depicts the response curve of Ag@1/GCE to continuous addition of 1 mmol·L-1glucose and 0.1 mmol·L-1interference species.When 1 mmol·L-1glucose was firstly added,a significantly rising current response was observed,and the other kinds of interfer⁃ences caused the negligible current response.After adding 1 mmol·L-1glucose again,the current signal was the same as when adding glucose for the first time.These results indicated that Ag@1 sensor had excellent selectivity for the detection of glucose.

Fig.7 (a)I⁃t curve of Ag@1/GCE with successive addition of 1 mmol·L-1glucose and other 0.1 mmol·L-1interfering species;(b)Stability of Ag@1/GCE to 1 mmol·L-1glucose in 0.1 mol·L-1NaOH solution at 0.85 V for 2 000 s;(c)Changes of catalytic activity of the same Ag@1/GCE during 30 d of storage time

In addition,the long⁃term stability of this opti⁃mized sensor was also examined.As shown in Fig.7b,the current signal appeared tiny descend for a long period of 2 000 s.Moreover,Ag@1/GCE was stored for stability analysis as displayed in Fig.7c,and its activity was tested every 5 d.It should be noted that after Ag@1/GCE was stored for 10 d,the activity decreased by 6.85%,and it remained at the original 80.6%after 30 d.The good stability is due to the presence of Nafion film which significantly slowed down the peel of com⁃posite film from Ag@1/GCE surface,thus further mani⁃fested the stability and recyclable electrocatalytic activ⁃ity of Ag@1.As to the repeatability test,through recording the five sets of mean values of the current changes of the parallel samples on Ag@1/GCE,the relative standard deviation(RSD)was calculated to be 5.34%(Fig.S5).

3 Conclusions

In summary,a novel CP structure(1)with a PIAD2-ligand has been successfully synthesized firstly for the non⁃enzymatic sensor of glucose in a facile way.Here,the containing⁃Ag@1 suspension was adsorbed on the electrode surface directly,then the Nafion solu⁃tion was coated on the GCE surface and efficiently restrained the CPs fall⁃off.The as⁃prepared low⁃cost glucose sensor based on Ag@1 exhibited good sensitivi⁃ty,selectivity and stability.More interestingly,for Ag@1,the interference from the oxidation of common six in⁃terfering species such as AA and UA was effectively avoided.It is found that the Mn（Ⅱ）⁃based CPs can be used as immobilization hosts to load NPs in the glucose electrocatalysis.Furthermore,this research has great significance for exploring the application in electroca⁃talysis with poorly conductivity CPs as a catalytic ac⁃tive center and also provides a promising idea for the development of non⁃enzymatic electrochemical sensors.

Supporting information is available at http://www.wjhxxb.cn