Kannan Balasubramanian

Humboldt-Universität zu Berlin

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Klaus Kern

École Polytechnique Fédérale de Lausanne

Marko Burghard

Max Planck Institute for Solid State Research

Tilmann J. Neubert

Humboldt-Universität zu Berlin

Rodrigo M. Iost

Universidade Federal de São Carlos

Anur Yadav

Humboldt-Universität zu Berlin

Roman Sordan

Politecnico di Milano

Tetiana Kurkina

RWTH Aachen University

Alf Mews

Universität Hamburg

Laura Zuccaro

University of Toronto

Michel Wehrhold

Humboldt-Universität zu Berlin

Cooperative Institutions

Max Planck Institute for Solid State Research

Max Planck Society

École Polytechnique Fédérale de Lausanne

Humboldt-Universität zu Berlin

Politecnico di Milano

Centre National de la Recherche Scientifique

Universidade de São Paulo

University of Rome Tor Vergata

Universidade Federal de São Carlos

Federal Institute For Materials Research and Testing

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Infrared and Raman spectroscopic analysis of functionalized graphene

Applied Research (2022)

Karsten Hinrichs Tilmann J. Neubert Christoph Kratz Timur Shaykhutdinov Jörg Rappich

Abstract In this mini‐review, we discuss our work in the analysis of material properties of electrochemically functionalized graphene using infrared (IR) and Raman techniques as thin film sensitive vibrational spectroscopies. Multiscale characterization is demonstrated using a combination of IR spectroscopic ellipsometry (IRSE), confocal Raman spectroscopy (RS) and photothermal atomic force microscopy coupled IR analysis (AFM‐IR). IRSE is used for spots with dimensions of a few mm, RS for spots with a diameter of a few micrometer and AFM‐IR at the sub‐100 nm scale. In the first part of the article, functionalized large‐area graphene sheets electrochemically modified by ultrathin oligomer films of maleimidophenyl or 4‐aminophenyl acetic acid are studied. Characteristic molecular vibrations of the functional layers are analyzed by the IR methods, while the graphene‐related phonons are characterized by RS. This dual approach allowed for the determination of the attached functional groups as well as the identification of the nature of chemical coupling of the functional moieties to graphene. The thickness of the deposited layers extracted from IRSE data correlates very well with AFM measurements. In the second part of the article, functionalized graphene sheets are characterized by correlative vibrational optical spectroscopy directly at a device level.

Micrometer

10.1002/appl.202200054

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Citations (2)

Electrochemistry at the edge of a van der Waals heterostructure

Aleksandra Plačkić Tilmann J. Neubert Kishan Ashokbhai Patel Michel Kuhl Kenji Watanabe

Artificial van der Waals heterostructures, obtained by stacking layered two-dimensional materials, represent a novel material platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional edge of a graphene sheet, which is sandwiched between two hexagonal boron nitride (hBN) multilayer flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is protected and isolated by the hBN stack, and the edge of the graphene sheet is exclusively available in the solution. This forms an electrochemical nanoelectrode, which enabled the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The relatively low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V/s). Using fast scan cyclic voltammetry imaging, redox species could be detected voltammetrically down to micromolar concentrations with sub-second time resolution at the sandwiched graphene edge, promoted by the rapid equilibration of analyte species in the diffusion layer. Furthermore, the nanoband nature of the edge electrode allows its operation directly in water in the absence of added electrolyte. Finally, two adjacent edge electrodes could be realized in a redox-cycling format. In all, the van der Waals edge electrode is unique among nanoelectrodes as it enables investigations of all the above-mentioned phenomena in the same device. Due to its versatility, it constitutes a new avenue for nanoscale electrochemistry, which will be useful for studying electron transfer mechanisms as well as for the detection of analyte species in ultralow sample volumes.

10.26434/chemrxiv-2023-hbqjd-v2

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pH Sensitivity of Edge-Gated Graphene Field-Effect Devices with Covalent Edge Functionalization

ACS Applied Electronic Materials (2022)

Tilmann J. Neubert Janina Krieg Anur Yadav Kannan Balasubramanian

We introduce here a strategy for a field-effect device, termed graphene edge field-effect transistor (GrEdge-FET), where a micron-wide graphene monolayer is gated exclusively through its edge in an aqueous environment. This is achieved by passivating the basal plane selectively using photolithography. We observe a field-effect behavior in buffer solutions with an ON/OFF ratio of nearly 10 in a small gate-voltage range (±0.5 V) without any need for complex nanofabrication or specialized electrolytes. We attribute this effect to the electrical double layer capacitance at the edge–electrolyte interface, which efficiently gates the entire graphene sheet although it acts only at the edge. We demonstrate that GrEdge-FET devices find applications as pH sensors. Through diazonium electrochemistry, the edges are functionalized persistently with substituted phenyl moieties, which renders the devices with a higher pH sensitivity than classical graphene FETs. Moreover, since only the edge is modified, the favorable field-effect behavior is preserved, despite the covalent nature of the attachment of the functional groups.

Field effect

10.1021/acsaelm.2c00880

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Fast Electron Transfer Kinetics at an Isolated Graphene Edge Nanoelectrode with and without Nanoparticles: Implications for Sensing Electroactive Species

ACS Applied Nano Materials (2020)

Anur Yadav Michel Wehrhold Tilmann J. Neubert Rodrigo M. Iost Kannan Balasubramanian

We present here the electrochemistry at a photolithographically created isolated monolayer graphene edge (GrEdge). The millimeter-long GrEdge is found to behave like a nanowire, exhibiting very high mass transport rates, characteristic of nanoelectrodes. Accordingly, the voltammetric response at such electrodes is dictated by the kinetics of heterogeneous electron transfer (HET). We observe high electron transfer rates at GrEdge electrodes, at least 14 cm/s for the outer-sphere probe ferrocenemethanol and 0.06 cm/s or higher for the inner-sphere probe Fe(CN)63–. Upon selective modification of the edge with gold nanoparticles, the HET is found to be reversible, with the voltammetric curve showing a typical mass-transport-limited Nernstian response for both kinds of probes. Subsequently, the electrodes are evaluated as electrochemical sensors for the detection of reduced form of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD). The nanoscale geometry with a unique diffusional profile of the unmodified GrEdge enables the sensing of NADH down to micromolar concentrations. Taken together, our simple strategy for the realization of graphene edge electrodes enables the availability of a versatile high-aspect one-dimensional nanoelectrode with the capability to study fast electron transfer kinetics. Moreover, such electrodes allow for facile detection of small amounts of electroactive species and hence will find applications in chemical sensing and biosensing.

Flavin adenine dinucleotide

10.1021/acsanm.0c02171

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Citations (19)

Raman properties of gold nanoparticle-decorated individual carbon nanotubes

Applied Physics Letters (2007)

Tilman Assmus Kannan Balasubramanian Marko Burghard Klaus Kern Matteo Scolari

Single-wall carbon nanotubes decorated by gold nanoparticles with sizes of a few tens of nanometers were investigated by confocal Raman microscopy. It was found that individual nanoparticles impart a sizable Raman enhancement exceeding one order of magnitude, without appreciably interfering with polarization dependent Raman measurements. By contrast, cavity effects within small nanoparticle agglomerates resulted in a 20-fold stronger enhancement and significant distortions of the polarization characteristic.

Nanometre

Agglomerate

10.1063/1.2731662

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Citations (32)

Real-Time Monitoring of Cell Adhesion onto a Soft Substrate by a Graphene Impedance Biosensor

ACS Sensors (2023)

Victoria Guglielmotti Emil Fuhry Tilmann J. Neubert Michel Kuhl Diego Pallarola

Soft substrates are interesting for many applications, ranging from mimicking the cellular microenvironment to implants. Conductive electrodes on such substrates allow the realization of flexible, elastic, and transparent sensors. Single-layer graphene as a candidate for such electrodes brings the advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate, namely, poly(dimethylsiloxane) (PDMS). We have systematically studied the effect of surface energy before, during, and after the transfer of graphene. Thus, we have identified a suitable support polymer, optimal substrate (pre)treatment, and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors on a millimeter scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements, we infer that the softness of the substrate is slightly affected after the graphene transfer. However, we can modulate the stiffness using PDMS with differing compositions. Finally, we show that graphene sensors on PDMS can be successfully used as soft electrodes for real-time monitoring of the cell adhesion kinetics. The routine availability of single-layer graphene electrodes on a soft substrate with tunable stiffness will open a new avenue for studies, where the PDMS–liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity, and transparency.

Polymer substrate

10.1021/acssensors.3c01705

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Citations (2)

Biosensors based on carbon nanotubes

Analytical and Bioanalytical Chemistry (2006)

Kannan Balasubramanian Marko Burghard

Biomolecule

10.1007/s00216-006-0314-8

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Citations (714)

Real-time monitoring of cell adhesion on to a soft substrate by a graphene impedance biosensor

Victoria Guglielmotti Emil Fuhry Tilmann J. Neubert Michel Kuhl Diego Pallarola

Soft substrates are interesting for a range of applications from mimicking cellular micro-environment to implants. Conductive electrodes on soft substrates open a broad spectrum of possibilities such as electrical and electrochemical sensing coupled with the flexibility, elasticity and transparency of the underlying substrate. Single layer graphene on a soft substrate as a candidate for such flexible electrodes brings the additional advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate namely poly(dimethylsiloxane) (PDMS). Specifically, we have systematically studied the effect of surface energy before, during and after the transfer of graphene. Based on this, we have identified a suitable support polymer, optimal substrate (pre-) treatment and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors in millimeter-scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements with an AFM, we infer that the softness of the substrate is slightly affected after graphene transfer. However, we can modulate the stiffness using PDMS of differing composition. Finally, we show that graphene sensors on PDMS can be successfully used as electrodes for real-time monitoring of cell adhesion kinetics on a soft substrate. The routine availability of single layer graphene electrodes on a flexible soft substrate with tunable stiffness will open a new avenue for a range of studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity and transparency.

Polymer substrate

10.26434/chemrxiv-2023-6f0d9

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Citations (1)

Implementing a Secure E-Commerce Web Site

Advances in information security, privacy, and ethics book series (2016)

Kannan Balasubramanian

The design of a secure e-commerce website, involves process of grouping your systems together in common areas as defined by their requirements for security. These groupings or security zones will be regulated by the control systems (such as firewalls and routers) that you deploy in your site. They will also be monitored against attack by intrusion detection systems (IDSs) and other tools deployed within your environment. The main steps in securing the E-commerce Web Site are: (i) implementing Security Zones, (2) Deploying Firewalls, (3) Deciding Where to place the Components (4) Implementing Intrusion Detection (5) Managing and Monitoring the Systems.

Web site

10.4018/978-1-5225-0273-9.ch007

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Prevention of SQL Injection Attacks in Web Browsers

Advances in information security, privacy, and ethics book series (2016)

Kannan Balasubramanian

Applications that operate on the Web often interact with a database to persistently store data. For example, if an e-commerce application needs to store a user's credit card number, they typically retrieve the data from a Web form (filled out by the customer) and pass that data to some application or script running on the company's server. The dominant language that these database queries are written in is SQL, the Structured Query Language. Web applications can be vulnerable to a malicious user crafting input that gets executed on the server. One instance of this is an attacker entering Structured Query Language (SQL) commands into input fields, and then this data being used directly on the server by a Web application to construct a database query. The result could be an attacker's gaining control over the database and possibly the server. Care should be taken to validate user input on the server side before user data is used.

SQL Injection

Data Transformation Services

User-defined function

Autocommit

10.4018/978-1-5225-0273-9.ch011

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