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Journal ArticleDOI

A New Graphene Quantum Dot Sensor for Estimating an Antibiotic Concentration

01 Mar 2018-MRS Advances (Springer International Publishing)-Vol. 3, Iss: 15, pp 825-830
TL;DR: In this paper, a new analytical method for sensing ciprofloxacin (CPFX) antibiotic using GQD electrode in differential pulse voltammetry (DPV) which is based on the ferric ion interaction with CPFX is reported.
Abstract: The graphene quantum dots (GQD) are unique for several different applications especially in the area of sensors as they provide a platform for large surface area on which sensing material can be attached. We wish to report here a new analytical method for sensing ciprofloxacin (CPFX) antibiotic using GQD electrode in differential pulse voltammetry (DPV) which is based on the ferric ion interaction with CPFX. Ferric ion undergoes a well defined one electron reduction at GQD electrode in DPV at Ep=0.310 V vs saturated calomel electrode (SCE) with a peak width of 0.100 V. When nanomolar to micromolar concentrations of CPFX is present in the electrolytic bath, the ferric ion reduction peak decreases with the appearance of three new peaks at EpI=0.200 V, EpII=0.050 V and EpIII= -0.085V. The three peaks are attributed to the three stages of binding of CPFX with three positive charges of ferric ion. The decrease of the ferric ion peak at 0.31 V is proportional to the concentration of CPFX. Due to large surface area of GQD, the CPFX bound ferric ion shows enhanced currents in comparison to glassy carbon electrode. The sensor is fabricated by depositing GQD containing known concentration of ferric ion. The sensor response to different concentrations of CPFX is measured for an analytical purpose.

Summary (10 min read)

Jump to: [A Thesis Submitted in Partial Fulfillment of the][1.1 Graphene][1.2 Graphene Quantum Dots][1.3 Ciprofloxacin – Structure and significance][1.4 Analyte detection and sensing methods][1.5 Electrochemical sensing methods][1.6 Aim and scope of the thesis][2.1 Chemicals][2.2 Materials for experimentation][2.4.1 Basic principle][2.4.2 Experimental set-up][2.5 Electrochemical detection][2.5.1 Differential Pulse voltammetry (DPV)][2.5.2 Cyclic voltammetry (CV)][2.5.3 Chronoamperometry][2.6.1 Construction][2.6.3 Procedure for depositing the active material][2.6.4 Procedure for testing the sensor reproducibility][2.7.1 Basic Principle][2.7.2 Experimental set-up][2.8.1 Basic Principle][2.8.2 Experimental set-up][3.1 Concentration determination of CP stock solution][3.2.1 Area calibration of GCE modified by GQD][3.2.2 DPV analysis of CP with ferric ion][CP Concentration (µM) Peak current (µA)][3.2.4 CP concentration measurement by Chronoamperometry][3.2.5 Effect of pH in CP determination][3.3 Fluorescence measurements as a support of binding][3.3.1 Fluorescence of CP and ferric ion][3.3.2 Fluorescence of GQD and ferric ion][3.4 FTIR Spectroscopy in support of binding][3.5 Resistance measurements for determining CP concentration][1 and 2)][3.5.2 Resistance (Ri) measurements of the sensor blank][3.5.3 Resistance (Rf) measurements in different CP solutions][3.5.5 Effect of interferences][3.5.6 Effect of oxygen][3.5.7 Effect of Argon][5.1.1 Investigation using CNTs modified by GQD][5.1.2 Investigation of p-Aminophenol (PAP)][5.1.2.1.1 Effect of pulse size variation] and [5.1.2.1.2 Effect of step size variation]

A Thesis Submitted in Partial Fulfillment of the

  • Requirements for the Degree of Master of Science in Materials Science and Engineering.
  • Dr. Massoud Miri Date Associate Professor, School of Chemistry and Materials Science.
  • The approach adopted in developing resistive sensor is shown below.
  • Interdigitated gold electrodes 2. GQD bound interdigitated gold electrodes 3. Ferric ion bound to GQD 4. iii.

1.1 Graphene

  • Graphene is a two-dimensional (2D) flat monolayer made up of carbon atoms tightly packed into a honeycomb lattice [7].
  • Figure 1.2C refers to the 3D epitaxial growth of graphitic layers on top of other crystals [9].
  • The optical and electrical properties of graphene are very unique [11].
  • Graphene also exhibits excellent thermal and mechanical properties.
  • The high thermal conductivity of graphene is mainly because of the presence of holes in its structure, which allow phonons to pass through unimpeded.

1.2 Graphene Quantum Dots

  • Development of quantum mechanics has driven many investigations for solving the puzzle of atomic spectra [16].
  • The electrons of the artificial atoms are confined to a quantum well and are called quantum dots (QDs) [17].
  • Because of incredibly small pieces of graphene of size less than 30nm, GQDs possess highly superior properties like large diameter, high surface area and better surface grafting.
  • The main research breakthrough in GQD production, by both top-down and bottom-up approaches is finding an efficient process that can take GQD synthesis to an industrial level and making efficient use of the exceptionally outstanding properties of GQD.
  • GQDs have been investigated to develop electronic, photo-luminescent (PL), electrochemical and electrochemiluminescent (ECL) sensors.

1.3 Ciprofloxacin – Structure and significance

  • Antibiotics are regarded as clinically safe and well-tolerated class of antimicrobials [38], however, they may have very serious negative side-effects.
  • Ciprofloxacin (CP) is a second generation fluoroquinolone antibiotic which has a broad activity spectrum for killing the bacteria [40].
  • The development of quinolone structures has been described along two parallel 11 pathways, the naphthyridones and fluoroquinolones [40].
  • The molar mass of CP is 337.341 g/mol and generally exists in crystalline powder form generally [41].
  • Overdose of CP may result in risk of worsening muscle weakness, tendon rupture, renal toxicity, cerebral thrombosis, chest pain, myocardial infarction, cardiopulmonary arrest, etc. Antibiotic-tolerant populations include both intrinsically resistant organisms and bacteria that acquired genetic determinants able to confer resistance [44].

1.4 Analyte detection and sensing methods

  • Some of the above mentioned techniques are used for detecting and measuring CP concentration.
  • Samanidou et al [48], developed a direct method for the determination of four fluoroquinolones like CP, norfloxacin, ofloxacin and enoxacin in human blood serum samples by using HPLC technique.
  • This method involved packing of Sephadex SP C-25 cation-exchange gel beads in a flow cell and continuous monitoring of its native absorbance on the solid phase at 277 nm.
  • In a study [53], utilizing chemiluminescence (CL) for CP determination, CP in biological fluids and in CP hydrochloride tablet and injection has been determined by a novel, rapid, sensitive and analytical method.

1.5 Electrochemical sensing methods

  • Identification of analytes and determination of organic and inorganic compounds by electroanalytical techniques has been very popular among scientists and researchers [46].
  • Electrochemical techniques have many advantages over the other conventional analytical techniques [46].
  • Yan et al. [54], developed a photo-electrochemical detecting platform for CP detection in their study.
  • Excellent photoabsorption property and photoelectric conversion efficiency in visible region was exhibited by the Bi/BiOBr composites.
  • All the above mentioned studies [45], [55]-[58], have a limitation of reproducibility due to the direct electrochemical oxidation of CP.

1.6 Aim and scope of the thesis

  • As described in the above literature review [6]-[37], the novel materials graphene and GQDs have exceptionally outstanding properties and attributes useful for sensor functioning.
  • Two sensor methodologies for CP concentration determination were formulated: 1. Electrochemical sensor (formulated by an electrochemical technique called Differential Pulse Voltammetry (DPV)); and 2.
  • There has been no study so far that has proposed a method to bind CP to ferric ion and to determine CP concentration by monitoring the electro-reduction behavior of the bound species, thus making the research unique.
  • The sensor mechanism is developed based on the change in resistance of the sensor with respect to the concentration of CP.
  • The fourth chapter provides the conclusion with emphasis on the merits and demerits of the study.

2.1 Chemicals

  • The following Table 2.1. provides the details about the different chemicals used in the study.
  • Acetic acid CH3COOH 60.05 Macron Fine Chemicals 96 Sodium acetate CH3COONa 82.03 Sigma-Aldrich Chemicals company 95 Argon gas Ar 39.948 Linde, Fulton, NY 99.9 Oxygen gas O2 32 Linde, Fulton, NY 99.9 GQD used in this study was manufactured by a US patented method [59], where GQD is produced in a single step process involving no secondary purifications.
  • The process utilizes an electrochemical cell containing electrodes with variable gaps including a zero gap, containing an anode electrode including graphite, a cathode electrode including electrically conductive material with an electrolyte-free electrochemical bath including water and an organic liquid that produces joule heating along with oxygen embrittlement.

2.2 Materials for experimentation

  • The Table 2.2. given below provides the experimental materials used with the names of the company from which they were procured.
  • Counter electrode – Graphite Gamry Instruments Screen Printed electrode (SPE) Gamry Instruments Fluorescence cuvette Fireflysci cuvette shop UV-Visible absorption cuvette Fireflysci cuvette shop Inter-digitated gold electrode sensor Electronics Design Center (Case Western, Ohio) Pipettes, burettes, emery paper, volumetric flasks, scintillation vials, beakers and graduated cylinders A-level stockroom of School of Chemistry and Materials Science, College of Science, RIT.

2.4.1 Basic principle

  • The UV-Visible absorption spectroscopy is based on the absorption of ultra-violet (UV) or visible radiation by the species under investigation [60].
  • The absorption is a two-step process: 1. Electronic excitation of the species M + hʋ M* (2.1) where, M represents the molecular species under investigation, hʋ represents the energy of the photon and M* represents the absorption product or the electronically excited species.
  • This photochemical transition is measured in the UV-visible absorption spectroscopy experiment.

2.4.2 Experimental set-up

  • UV-visible absorption spectroscopy was used to calculate the concentration of CP solutions.
  • Hence, the CP solutions, whose concentration was calculated by weighing the gram equivalent weight of CP solid, could not be used because of the turbid and unclear nature of the solutions.
  • The turbid solutions were filtered until a clear solution was obtained.
  • The schematic of Shimadzu UV-2501PC – High Resolution Spectrophotometer and the quartz cuvette in which the measurements were taken are shown in Figure 2.1. below: 31 b. 1cm Quartz cuvette to study absorption.
  • The measurements were carried out by putting the solutions in a 1cm quartz cuvette where two sides of the cuvette were fully transparent to light and the other two sides completely opaque to light.

2.5 Electrochemical detection

  • The electrochemical studies were carried out using three different techniques like differential pulse voltammetry (DPV), cyclic voltammetry (CV) and chronoamperometry to investigate the behavior of CP and ferric ion using working electrodes like bare GCE (diameter= 0.3cm, length=6in, area=0.071cm2) and GQD modified GCE working electrodes (diameter=0.3cm, length=6 in, area= 0.071 cm2).
  • Electrochemical measurements were done with CNT pipette electrodes as working electrodes.
  • The electrodes were washed well with tap water and distilled water after cleaning.
  • The Figure 2.2. below shows all the different electrodes and GQD used to modify GCE in the study.
  • The high-resolution TEM images of GQD is shown in Figure 2.3 below. 33 improve visibility.

2.5.1 Differential Pulse voltammetry (DPV)

  • DPV is an effective electrochemical technique, which measures trace levels of organic and inorganic species [62] and consists of fixed magnitude pulses superimposed on a linear potential ramp applied to the working electrode.
  • The current is sampled twice in the DPV technique – 1.
  • The charging current decays exponentially, but the faradaic current (for a diffusion-controlled current) decays as a function of 1/(time)1/2.
  • The current response usually consists of current peaks as shown below in Figure 2.7.
  • The peak potential Ep, is characteristic to the electro-active species under investigation.

2.5.2 Cyclic voltammetry (CV)

  • CV is a very powerful electrochemical technique used for investigating the reduction and oxidation processes of atomic or molecular species [62].
  • CV is also extensively used for studying electron transfer-initiated chemical reactions, which includes catalysis.
  • It is popular in qualitative electrochemical studies because of rapidly locating the redox potential for the electroactive species [62].
  • The current resulting from the applied potential is measured and plotted in the cyclic voltammogram.
  • The chemical reversibility is used to denote the stability of the analyte.

2.5.3 Chronoamperometry

  • Chronoamperometry is an electrochemical technique used for studying processes like kinetics of chemical reactions, diffusion and adsorption [62].
  • A step potential is applied to the electrode and the resulting current vs. time is observed.
  • The potential step applied and the response to the potential step is similar to Figure 2.10.
  • Capacitive current depends on the charging up of the electrode capacitive layer and Faradaic current depends upon diffusion of the electroactive species.
  • The resulting plot of chronoamperometry experiment consists of a plot of resulting current vs. time similar to Figure 2.11.

2.6.1 Construction

  • A new sensor was constructed by using interdigitated finger electrode (5x5 mm) on 0.6 mm thick alumina substrate.
  • The interdigitized arrangement is shown below in Figure 2.12.
  • The two holes were used for connecting the insulated copper wires.
  • This required extreme caution to prevent the lead solder to flow into the inter-digitated space.
  • The rear side soldering of the sensor and the front view of the sensor after soldering is shown in Figure 2.13.

2.6.3 Procedure for depositing the active material

  • The active material is made of graphene quantum dots (GQD) and ferric chloride.
  • The deposition is done by dropwise addition of two drops of GQD first on the inter digitized space and later by the addition of 50µL drops of 0.1M ferric chloride.
  • The sensor was air dried for a couple of hours to ensure that a good coating of the active material was obtained.

2.6.4 Procedure for testing the sensor reproducibility

  • A test chamber and resting chamber were constructed for the measurements that would be conducive to CP.
  • The test chamber and resting chamber are shown in Figure 2.14.
  • The testing chamber was a 250-ml tall form beaker fitted with Teflon cork having holes for injecting the analyte and for inserting the interdigitated sensor.
  • After placing the sensor in the testing chamber, the two wires were taken out through the holes and connected to a digital multimeter that is interfaced with computer through RS230.
  • The meter view software was used for the measurements.

2.7.1 Basic Principle

  • Fluorescence spectroscopy is based on the principle of photo-luminescence (PL), which is the emission of an absorbed radiant energy in the form of light [60].
  • Generally, the emitted light is almost of longer wavelength than that of the absorbed radiation.
  • The most important selection rule for all PL systems is that spin/multiplicity must not change during an electronic transition.
  • The equation for multiplicity is given by Multiplicity = 2S+1 where S is the total spin angular momentum.
  • The fluorescence spectrum is obtained by 45 measuring fluorescence intensity at varying wavelengths while the excitation wavelength is constant.

2.7.2 Experimental set-up

  • Fluorescence spectroscopy was used in this study to investigate the fluorescence behavior of the solutions of: 1. Different concentrations of ferric ion in a constant concentration (reference sample) of CP, and 2.
  • Different concentrations of ferric ion in a constant concentration (reference sample) of GQD.
  • The schematic of Horiba Jobin Yvon Fluoromax-4 Fluorimeter and the quartz cuvette in which the fluorescence measurements were taken are shown in Figure 2.16.
  • Based on the changes in intensities and wavelengths of peaks seen in fluorescence spectra of different solutions, various inferences are made and presented in section 3.4 of this thesis.

2.8.1 Basic Principle

  • The FTIR spectroscopy is one of the most useful techniques for identifying organic and inorganic chemicals [60] by identifying types of chemical bonds in a molecule due to the infrared absorption spectrum that is like a molecular "fingerprint".
  • The wavelength of light absorbed is characteristic of the type of the chemical bond in the molecule.
  • The basic principle of FTIR depends on the vibration of molecular bonds at various frequencies.
  • According to quantum mechanics, these frequencies correspond to the ground state (lowest frequency) and several excited states (higher frequencies).
  • The molecular vibration can be increased by exciting the bond by having it absorb light energy.

2.8.2 Experimental set-up

  • FTIR was performed for solutions like ferric ion only (ferric chloride), and combination of ferric ion and CP.
  • The schematic of Biorad Excalibur Series FTS 300 on which the FTIR measurements were performed are as shown in the Figure 2.17.
  • The background emission spectrum of the IR source (in atmospheric air) is first recorded, followed by the emission spectrum of the IR source with the sample in place.
  • Each FTIR recording comprises of 2000 scans in the range from 4000 cm-1 to 300 cm-1. Various inferences made from the absorption spectrum are illustrated in section 3.5.

3.1 Concentration determination of CP stock solution

  • The CP stock solutions were filtered and its concentration was measured by UV-visible absorption spectroscopy.
  • The solution was filtered and 0.1ml of the filtered solution was diluted to 10ml with distilled water, i.e, the filtered solution was diluted by a hundred times.
  • This stock solution of CP was used and diluted to make other CP solutions subsequently used in the study.

3.2.1 Area calibration of GCE modified by GQD

  • The GCE modified by GQD was made by coating the electrode surface of bare GCE with 30µL of electrochemically synthesized GQD and drying it in the oven at a temperature of 75ºC for about an hour.
  • The cyclic voltammogram obtained is shown in Figure 3.3.
  • By reports in the literature, it is known that if the peak separation voltage, ΔEp > 60mV, then the reaction is classified as a quasi-reversible reaction [60], where current is controlled by both the charge transfer and mass transport.
  • The plot of i(t) vs t-1/2 for the data obtained by the chronoamperometry curve is shown in Figure 3.5.
  • This result proves that the entire geometrical area of unmodified GCE became electrochemically active in the GCE modified by GQD.

3.2.2 DPV analysis of CP with ferric ion

  • The DPV experiment was performed using an electrolyte of 10ml - 0.1M Na2SO4 (pH=6.4) containing 5mM ferric ion in the form of 5mM FeCl3 solution.
  • Both the unmodified GCE and GCE modified by GQD electrodes were used to study the effect of CP on ferric ion.

CP Concentration (µM) Peak current (µA)

  • 61 Using this calibration curve, if any sample solution of unknown CP concentration is given, its value can be electrochemically determined by measuring the reduction peak current of [Fe3+ - (CP-)3] by DPV using GCE modified by GQD.
  • The sensitivity of this technique was observed to be about 243 nA/µM.

3.2.4 CP concentration measurement by Chronoamperometry

  • The Chronoamperometry experiment was performed using an electrolyte of 10ml - 0.1M Na2SO4 (pH=6.4) containing 5mM ferric ion in the form of 5mM FeCl3 solution.
  • The reverse potential step had a final potential of 0.6 V which was applied for 60s.
  • 63 As seen from Table 3.3 and Figure 3.14, the total integrated coulombic charge decreases with increasing CP concentration.
  • The decrease in the ferric ion coulombic charge is due to the decrease in ferric ion concentration because of the formation of [Fe3+ - (CP-)3].
  • The calibration curve for the sensor shown in Figure 3.14.

3.2.5 Effect of pH in CP determination

  • To study the effect of pH on the measurement of CP concentration, the DPV experiment was performed using an electrolyte of 10ml – 0.1M Na2SO4 and acetate buffer (pH=5.6) containing 1.2mM ferric ion in the form of 1.2mM FeCl3 solution.
  • Three different concentrations (2.64µM, 4.4µM and 6.09µM) of CP solutions were added to the system containing 1.2mM ferric ion.
  • DPV was performed by scanning the system from an initial voltage of 0.6 V to a final voltage of -0.4 V by applying fixed magnitude pulses, each of 20mV pulse size, 0.1s pulse time and 1s sample time.
  • The differential pulse voltammogram of 1.2mM ferric ion obtained is shown in Figure 3.15.
  • The DPV curve obtained when various CP concentrations were added to the system is shown in Figure 3.16.

3.3 Fluorescence measurements as a support of binding

  • The experiments were carried with the following solutions: 66 1. Different concentrations of ferric ion in a constant concentration (reference sample) of CP, and 2.
  • Different concentrations of ferric ion in a constant concentration (reference sample) of GQD.
  • An excitation wavelength of 350 nm was used for sample excitation in these experiments.
  • The fluorescence emission of CP and ferric ion solutions were monitored by scanning the wavelength range from 380 nm to 600 nm.
  • The fluorescence emission of GQD and ferric ion solutions were studied by scanning a wavelength range from 355 nm to 600 nm.

3.3.1 Fluorescence of CP and ferric ion

  • The reference sample used while studying the fluorescence emission of CP and ferric ion solutions was 24.3 µM CP solution.
  • Excited state interaction, or b. Ground state interaction.
  • The absorption spectra is shown in Figure 3.19.
  • This result concluded the absence of excited state interaction of CP and ferric ion.
  • The unbound CP contributed to the fluorescence of the solution in the mixture whose concentration decreases with increasing concentration of ferric ion in the medium.

3.3.2 Fluorescence of GQD and ferric ion

  • Fluorescence experiments also provided additional support to indicate the binding of Fe3+ to GQD.
  • The fluorescence emission spectra of reference GQD sample and different concentrations of ferric ion in a constant GQD concentration are shown in the Figure 3.20.
  • The ferric ion binding to GQD resulted in the decrease in the counts as shown in Figure 3.21.
  • These observations along with FTIR ( see section 3.4) confirmed the binding of Fe3+ to GQD and hence acts as the support to the utility of the Fe3+- GQD sensor approach for determining CP concentration (as discussed in section 3.2.3.2, 3.2.4 and 3.5).

3.4 FTIR Spectroscopy in support of binding

  • FTIR spectroscopy was done on solids with ferric chloride alone and a mixture of ferric chloride with added CP solution.
  • This was carried out to have additional support to the binding mechanism of anionic CP to positively charged ferric ion Fe3+.
  • Table 3.4 gives the FTIR features of the ferric ion and the bound ferric ion with CP.

3.5 Resistance measurements for determining CP concentration

  • The active material of the sensor is made of GQD and ferric chloride.
  • The deposition is done by dropwise addition of two drops of GQD first on the inter digitized space and later by the addition of 50µL drops of 0.1M ferric chloride.

1 and 2)

  • For the types of plots 1 and 2, the original resistance of the sensor in air (that is the resistance measured for the active material before treating with CP) remains the same.
  • The resistance change with each concentration is calculated by keeping Ri constant.
  • For the 3rd type of plot, the resistance of the sensor, Ri keeps changing as the available sites decreases continuously.
  • The results of measurements performed using the inter-digitated gold electrode sensor with GQD-ferric ion active material is discussed in the following sections.
  • All the resistance and concentration plots for calibration have been plotted by ignoring the decreasing available sites concept.

3.5.2 Resistance (Ri) measurements of the sensor blank

  • The resistance measurements were made using the interdigitated Gold electrode with GQD bound ferric ion.
  • Three resistance measurements were made by repetitively switching the interdigitated GQD-ferric ion sensor between the resting chamber for 30 minutes and testing chamber.
  • These three resistance measurements are made in the testing chamber as the sensor blank with a finite number of active sites, Ri. Table 3.5 gives the results of these three measurements.
  • The sensor blank also showed the property of giving 267.4 mV output when measured in the configuration Au/Al2O3: GQD-Fe3+/Au (3.6) in the testing chamber.

3.5.3 Resistance (Rf) measurements in different CP solutions

  • Resistance of the sensor in different solutions of 5 different CP concentrations was measured by injecting the solution into the sensor testing chamber and allowing the resistance to reach a stable value.
  • After each measurement, the sensor was washed well with distilled water and kept in a hot bath for 15 minutes to desorb any species adsorbed on the sensor surface.
  • 78 As seen from Table 3.6, the resistance of the sensor increases with increasing CP concentration.
  • This validates the equation 3.4 where the resistance increases with decreasing number of available Fe3+ sites and provides support to the binding reaction discussed in section 3.2.2.1 implying that [Fe3+ - (CP-)3] provides higher resistance to voltage.

3.5.5 Effect of interferences

  • 81 The response of the sensor to different CP solutions containing 0.5M Urea is shown in Table 3.8.
  • It also validates the equation 3.5 where resistance increases with decreasing number of available Fe3+ sites.
  • To obtain statistically accurate measurements, each resistance measurement in the study was repeated for 3 trials.

3.5.6 Effect of oxygen

  • When the sensor blank was kept in the testing chamber, oxygen (air) was flown into the chamber to record the sensor response.
  • When the gas is desorbed, the sensor regained its original resistance value.

3.5.7 Effect of Argon

  • When the sensor blank was kept in the testing chamber, argon gas was flown into the chamber to record its response.
  • The sensor regained its original resistance value when the adsorbed gas was desorbed.
  • While using unmodified GCE as the working electrode to determine CP concentration, the DPV calibration curve plotted by measuring the net peak current of ferric ion at 0.34V is used to measure the concentration of CP.
  • The sensitivity of this technique was about 55nA/µM.
  • Some of the preliminary results obtained with CNT pipette electrodes modified by GQD for studying CP and with GCE modified by GQD for investigating PAP are given below.

5.1.1 Investigation using CNTs modified by GQD

  • Electrochemical studies were carried out using CNT pipette electrodes supplied by the Nano Bio-Interface laboratory.
  • DPV was performed by scanning the system from an initial voltage of 0.6 V to a final voltage of - 86 0.5 V by applying fixed magnitude pulses, each of 20mV pulse size, 0.1s pulse time and 1s sample time.
  • Figure 5.1 shows that the unmodified CNT pipette electrodes were very poor in performance and could not give any valuable results.
  • The DPV curve of unmodified CNT pipette electrode modified by GQD is shown in Figure 5.2.. 87 As seen from Figure 5.2., there is a DPV peak observed at Ep = 0.31 V corresponding to ferric ion with a peak current of 66.28 nA.
  • The sensitivity and performance of CNT pipette electrodes modified with GQD is clearly better than unmodified CNT pipette electrodes.

5.1.2 Investigation of p-Aminophenol (PAP)

  • To electrochemically investigate the behavior of PAP, electrochemical experiments like CV and DPV were performed using an electrolyte of 10ml - 0.1M Na2SO4 (pH=6.4) containing different concentrations of PAP.
  • Both the unmodified GCE and GCE modified by GQD electrodes were used to study the behavior of PAP.
  • DPV was performed by scanning the system from an initial voltage of -0.3 V to a final voltage of 0.55 V by applying fixed magnitude pulses, each of 88 25mV pulse size, 2mV step size, 3s pulse time and 5s sample time.
  • The effect of varying different parameters of DPV like step size and pulse size while using unmodified GCE was also determined.
  • The results of different electrochemical experiments are described below.

5.1.2.1.1 Effect of pulse size variation

  • DPV was performed by scanning the system from an initial voltage of -0.3 V to a final voltage of 0.55 V by applying fixed magnitude pulses, each of 25mV pulse size, 3s pulse time and 5s sample time and varying step sizes.
  • The DPV curve for 1µM PAP with varying step sizes is shown in Figure 5.4..
  • As seen from Figure 5.4., the peak for 1 µM PAP at Ep=0.3 V became narrower with decreasing step size.
  • The most well defined, narrow peak was obtained for 2 mV step size.
  • Hence, the optimum parameter for step size was identified to be 2 mV.

5.1.2.1.2 Effect of step size variation

  • DPV was performed by scanning the system from an initial voltage of -0.3 V to a final voltage of 0.55 V by applying fixed magnitude pulses, each of 3s pulse time, 2mV step size and 5s sample time and varying pulse sizes.
  • The effect of varying pulse sizes was studied with respect to the optimized step size 2 mV.
  • As seen from Figure 5.5., the peak current at Ep = 0.3V for 1 µM PAP continuously increased with increasing pulse size.
  • The values of peak current for various pulse sizes and 2mV step size is shown in Table 5.1. 91 From Figure 5.5., the most well-defined, sharp, narrow peak with the largest peak current of was obtained for 2 mV step size.
  • Hence, the optimum parameters for step size was identified to be 2 mV and pulse size of 50mV for studying the DPV of 1µM PAP using unmodified GCE.

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R.I.T.
A New Graphene Quantum Dot Sensor for
Estimating an Antibiotic Concentration
by
Nuzhet Nihaar Nasir Ahamed
A Thesis Submitted in Partial Fulfillment of the
Requirements for the Degree of
Master of Science
in
Materials Science and Engineering.
School of Chemistry and Materials Science
College of Science
Rochester Institute of Technology
Rochester, NY
May 3
rd
2018

School of Chemistry and Materials Science
College of Science
Rochester Institute of Technology
Rochester, New York
CERTIFICATE OF APPROVAL
The M.S. degree thesis of Nuzhet Nihaar Nasir Ahamed has been examined and approved by the
thesis committee as satisfactory for the thesis required for the M.S. degree in Materials Science
and Engineering.
______________________________________________________________________________
Dr. K.S.V Santhanam Date
Professor, School of Chemistry and Materials Science. (Advisor)
______________________________________________________________________________
Dr. Gerald Takacs Date
Professor, School of Chemistry and Materials Science.
_____________________________________________________________________________
Dr. Massoud Miri Date
Associate Professor, School of Chemistry and Materials Science.
______________________________________________________________________________
Dr. Michael S Pierce (MSE Graduate Program Director) Date
Associate Professor of Physics, School of Physics and Astronomy.

i
ABSTRACT
The antibiotics have impacted the human ailments by curtailing the growth of microbes and by
providing relief from microbial diseases. While there are a large number of analytical methods
available for the determination of antibiotics concentration, they are time consuming and
impractical for usage in the fields. This thesis is aimed at overcoming the deficiencies of those
methods in developing a new sensor. It reports a study of graphene quantum dots (GQD) bound
ferric ion for sensing an antibiotic, ciprofloxacin (CP). The interaction of ferric ion with CP was
used as a probe for the analytical estimation of CP using differential pulse voltammetry (DPV). A
solution containing ferric ion exhibits a well-defined cathodic peak at E
pc
=0.310 V vs saturated
calomel electrode (SCE) with a peak width of 0.100V. When nanomolar to micromolar
concentration of CP is present in the solution, along with ferric ion, three new peaks at
E
pc
I
=0.200V, E
pc
II
=0.050 V and E
pc
III
=-0.085V are observed due to the binding of CP to ferric ion.
The decrease in peak current of E
pc
at 0.310 V is proportional to the concentration of CP in the
solution. The peak current at 0.200 V shows an increase corresponding to the CP concentration in
solution. These results paved the way for examining the prospectus for developing a portable
resistive sensor using interdigitated gold electrodes on alumina substrate. The principle of this
sensor is based on that ferric ion bound to GQD will have a finite resistance and when it is bound
to CP the resistance will increase as the charge transport faces a barrier due to bulky CP molecules.
With a view to establish that ferric ion is binding to GQD, fluorescence of GQD has been recorded
with ferric ion in solution.

ii
The approach adopted in developing resistive sensor is shown below.
The numbers in the above picture denotes
1. Interdigitated gold electrodes
2. GQD bound interdigitated gold electrodes
3. Ferric ion bound to GQD
4. Attachment of CP to ferric ion.
The sensor response is found to be dependent on the activity of the availability of ferric ion on
GQD resulting in the usage of it as a disposable sensor. The interference of urea in the
measurement of CP was examined for the practical usage of it in urine analysis.

Citations
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TL;DR: In this article, a glass nanopipette electrode with graphene quantum dots was constructed for in vitro interfacial bioelectrochemical studies, which can provide a mechanistic understanding of complex biological systems and cell heterogeneity.
Abstract: Single-cell analysis is an emerging technology that can provide a mechanistic understanding of complex biological systems and cell heterogeneity. Any disruption of its activity can be monitored through interfacial bioelectrochemistry. A new glass nanopipette electrode laden with graphene quantum dots (20–50 nm) has been constructed for in vitro interfacial bioelectrochemical studies. A platinum or copper wire (0.0006″ dia) was placed inside the glass nanopipette with a tip size of 1 mm which was subsequently covered with graphene by dip coating. The glass nanopipette has been characterized by X-ray fluorescence as containing Si (96.82%), K (2.65%), and Fe (0.20%). The suitability of the electrode for studies involving oxidative stress produced by p-aminophenol (PAP) that results in membrane disruption and the frequency of molecular attachment of PAP to graphene has been relevant to the understanding of cell disruption. In this context, the electrochemical oxidation of PAP has been probed in vitro through differential pulse voltammetry (DPV) using the glass nanopipette electrode. The new electrode shows promise for examining electroactive neurotransmitter during its functioning in chronic diseases.

1 citations

References
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Abstract: Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

35,293 citations

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TL;DR: In this article, a comparison was made for the adsorption capacity of ciprofloxacin (CPX) on three types of carbon-based materials: activated carbon, carbon nanotubes and carbon xerogel.

324 citations

Journal ArticleDOI
TL;DR: High-performance liquid Chromatographic methods for the analysis of fluoroquinolones in biological fluids are reviewed and sample preparation and handling procedures, chromatographic conditions, and detection methods are discussed.

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Journal ArticleDOI
TL;DR: The porous-Nafion-MWCNT/BDD electrode enhanced detection of CFX due to selective adsorption, which was accomplished by a combination of electrostatic attraction at -SO3(-) sites in the porous Nafion film and the formation of charge assisted hydrogen bonding between CFX and -COOH MWCNT surface functional groups.
Abstract: This study focuses on the development of electrochemical sensors for the detection of Ciprofloxacin (CFX) in natural waters and wastewater effluents. The sensors are prepared by depositing a layer of multiwalled carbon nanotubes (MWCNTs) dispersed in a porous Nafion film on to a boron-doped diamond (BDD) electrode substrate. The porous-Nafion-MWCNT/BDD electrode enhanced detection of CFX due to selective adsorption, which was accomplished by a combination of electrostatic attraction at −SO3– sites in the porous Nafion film and the formation of charge assisted hydrogen bonding between CFX and −COOH MWCNT surface functional groups. By contrast, the bare BDD electrode did not show any activity for CFX oxidation. The sensors were selective for CFX detection in the presence of other antibiotics (i.e., amoxicillin) and other nontarget water constituents (i.e., Cl–, Ca2+, humic acid, sodium dodecylbenzenesulfonate, salicylic acid, 4-aminobenzoic acid, and 4-hydroxybenzoic acid). A limit of detection of 5 nM (S/N...

100 citations

Journal ArticleDOI
16 Aug 2000-Talanta
TL;DR: A method for the determination of trace amounts of ciprofloxacin has been developed, based on solid-phase spectrofluorimetry, and was validated applying the standard addition methodology and using HPLC as a reference method.

87 citations