Alternating current electrohydrodynamics induced nanoshearing and fluid micromixing for specific capture of cancer cells

We report a new tuneable alternating current (ac) electrohydrodynamics (ac-EHD) force referred to as “nanoshearing” which involves fluid flow generated within a few nanometers of an electrode surface. This force can be externally tuned via manipulating the applied ac-EHD field strength. The ability to manipulate ac-EHD induced forces and concomitant fluid micromixing can enhance fluid transport within the capture domain of the channel (e.g., transport of analytes and hence increase target–sensor interactions). This also provides a new capability to preferentially select strongly bound analytes over onspecifically bound cells and molecules. To demonstrate the utility and versatility of nanoshearing phenomenon to specifically capture cancer cells, we present proof-of-concept data in lysed blood using two microfluidic devices containing a long array of asymmetric planar electrode pairs. Under the optimal experimental conditions, we achieved high capture efficiency (e.g., approximately 90%; %RSD=2, n=3) with a 10-fold reduction in nonspecific dsorption of non-target cells for the detection of whole cells expressing Human Epidermal Growth Factor Receptor 2 (HER2). We believe that our ac-EHD devices and the use of tuneable nanoshearing phenomenon may find relevance in a wide variety of biological and medical applications.

An alternative approach to selective sea water oxidation for hydrogen production

Sea water electrolysis is one of the promising ways to produce hydrogen since it is available in plentiful supply on the earth. However, in sea water electrolysis toxic chlorine evolution is the preferred reaction over oxygen evolution at the anode. In this work, research has been focused on the development of electrode materials with a high selectivity for oxygen evolution over chlorine evolution. Selective oxidation in sea water electrolysis has been demonstrated by using a cation-selective polymer. We have used a perm-selective membrane (Nafion®), which electrostatically repels chloride ions (Cl−) to the electrode surface and thereby enhances oxygen evolution at the anode. The efficiency and behaviour of the electrode have been characterized by means of anode current efficiency and polarization studies. The surface morphology of the electrode has been characterized by using a scanning electron microscope (SEM). The results suggest that nearly 100% oxygen evolution efficiency could be achieved when using an IrO2/Ti electrode surface-modified by a perm-selective polymer.

Mechanism of action of selenious acid in the electrodeposition of manganese

The effect of selenious acid as an addition agent in the electrodeposition of manganese was studied by analysing the current-potential curves for manganese deposition. The mechanism of action of this addition agent was found to be essentially similar to that proposed for sulphur dioxide, namely to affect the manganese deposition indirectly by influencing the hydrogen evolution reaction which is a parallel reaction at the electrode surface.

Polypyrrole based amperometric glucose biosensors

Biosensors have gained immense acceptance in the field of medical diagnostics, besides environmental, food safety and biodefence applications due to its attributes of real-time and rapid response. This synergistic combination of biotechnology and microelectronics comprises a biological recognition element coupled with a compatible transducer device. Diabetes is a disease of major concern since the ratio of world population suffering from it is increasing at an alarming rate and therefore the need for development of accurate and stable glucose biosensors is evident. There are many commercial glucose biosensors available yet some limitations need attention. This review presents a detailed account of the polypyrrole based amperometric glucose biosensors. The polymer polypyrrole is used extensively as a matrix for immobilization of glucose oxidase enzyme owing to its favourable features such as stability under ambient conditions, conductivity that allows it to be used as an electron relay, ability to be polymerized under neutral and aqueous mild conditions, and more. The simple one-step electrodeposition on the electrode surface allows easy entrapment of the enzyme. The review is structured into three categories (a) the first-stage biosensors: which report the studies from the inception of use of polypyrrole in glucose biosensors during which time the role of the polymer and the use of mediators was established. This period saw extensive work by two separate groups of Schuhmann and Koopal who contributed a great deal in understanding the electron transfer pathways in polypyrrole based glucose biosensors, (b) the second-stage biosensors: which highlight the shift of polypyrrole from a conventional matrix to composite matrices with extensive use of mediators focused at improving the selectivity of response, and (c) third-stage biosensors: the remarkable properties of nanoparticles and carbon nanotubes and their outstanding ability to mediate electrontransfers have seen their indispensable use in conjugation with polypyrrole for development of glucose biosensors with improved sensitivity and stability characteristics which is accounted in the review, which thus traces the evolution of polypyrrole from a conventional matrix, to composites and thence to the form of nanotube arrays, with the objective of addressing the vital issue of diabetes management through the development of stable and reliable glucose biosensors.

An EQCM Investigation of Electrochemical Precipitation of Mn(OH)(2) and Its Capacitance Behavior

Electrochemical quartz crystal microbalance (EQCM) has been used to study the electrochemical precipitation of Mn(OH)(2) on a Au crystal and its capacitance properties. From the EQCM data, it is inferred that NO3- ions get adsorbed on the Au crystal and then undergo reduction, resulting in an increase in pH near the electrode surface. Precipitation of Mn2+ occurs as Mn(OH)(2), with an increase in mass of the Au crystal. Mn(OH)(2) undergoes oxidation to MnO2, which exhibits electrochemical supercapacitor behavior on subjecting to electrochemical cycling in a Na2SO4 electrolyte. EQCM data indicate mass variations corresponding to surface insertion/extraction of Na+ ions during discharge/charge cycling. (C) 2010 The Electrochemical Society. DOI: 10.1149/1.3479665] All rights reserved.

Direct Growth of Gold Nanorods on Gold and Indium Tin Oxide Surfaces: Spectral, Electrochemical, and Electrocatalytic Studies

The surfactant-assisted seed-mediated growth method was used for the formation of gold nanorods (GNRs) directly on gold (Au) and indium tin oxide (ITO) surfaces. Citrate-stabilized similar to 2.6 nm spherical gold nanoparticles (AuNPs) were first self-assembled on ITO or Au surfaces modified with (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film and then immersed in a cationic surfactant growth solution to form GNRs. The growth of GNRs on the MPTS sol gel film modified ITO surface was monitored by UV-visible spectroscopy. The ITO surface with the attached spherical AuNPs shows a surface plasmon resonance band at 550 nm. The intensity of this absorption band increases while increasing the immersion time of the AuNP-modified ITO surface into the growth solution, and after 5 h, an additional shoulder band around 680 nm was observed. The intensity of this shoulder band increases, and it was shifted to longer wavelength as the immersion time of the AuNP-modified ITO surface into the growth solution increases. After 20 h, a predominant wave at 720 nm was observed along with a band at 550 nm. Further immersion of the modified ITO surface into the growth solution did not change the absorption characteristics. The bands observed at 550 and 720 nm were characteristics of GNRs, corresponding to transverse and longitudinal waves, respectively. The AFM images showed the presence of GNRs on the surface of the MPTS sol gel modified ITO surface with a typical length of similar to 100-120 nm and a width of similar to 20-22 nm in addition to a few spherical AuNPs, indicating that seeded spherical AuNPs were not completely involved in the GNRs' formation. Finally, the electrocatalytic activity of the surface-grown GNRs on the MPTS sol gel film modified Au electrode toward the oxidation of ascorbic acid (AA) was studied. Unlike a polycrystalline Au electrode, the surface-grown GNR-modified electrode shows two well-defined voltammetric peaks for AA at 0.01 and 0.35 V in alkaline, neutral, and acidic pHs. The cause for the observed two oxidation peaks for AA was due to the presence of both nanorods and spherical nanoparticles on the electrode surface. The presence of spherical AuNPs on the MPTS sol gel film oxidized AA at more positive potential, whereas the GNRs oxidized AA at less positive potential. The observed 340 mV less positive potential shift in the oxidation of AA suggested that GNRs are better electrocatalysts for the oxidation of AA than the spherical AuNPs.

In-situ formation of graphene-lead oxide composite and its use in trace arsenic detection

Reduced graphene oxide-lead dioxide composite is formed when EGO coated surface is electrochemically reduced along with lead ions in the solution. This composite has been shown to be an excellent material for low level detection of arsenic. Various functional groups present on EGO, in a wide pH range of 2-11, are responsible for the favorable interaction between metal ion and the modified electrode surface and subsequent trace level detection. X-ray photoelectron spectroscopy and Raman spectroscopic techniques confirm the formation of composite and its composition. Thin layer of lead dioxide along with reduced exfoliated graphene oxide has been shown to be responsible for the enhanced activity of the surface. The detection limit of arsenic is found to be 10 nM. This study opens up the possibility of using the composites for sensing applications and possibly simultaneous detection of arsenic and lead. (C) 2011 Elsevier B.V. All rights reserved.

EQCM investigation of electrochemical deposition and stability of Co-Pi oxygen evolution catalyst of solar energy storage

Photoassisted electrolysis of water is considered as an effective way of storing solar energy in the form of hydrogen fuel. This overall reaction involves the oxidation of water to oxygen at the anode and the reduction of protons to hydrogen at the cathode. Cobalt-phosphate-based catalyst (Co-Pi) is a potentially useful material for oxygen evolution reaction. In the present study, electrochemical deposition of Co-Pi catalyst is carried out on Au-coated quartz crystal from 0.1 M phosphate buffer (pH 7) containing 0.5 mM Co2+ ion, along with the simultaneous measurement of mass changes at the electrode surface. Cyclic voltammograms and mass variations are recorded during the course of deposition. A current peak is observed at 0.92 V vs Ag/AgCl, 3 M KCl corresponding to oxidation of Co2+ ion. The mass of the electrode starts increasing at this potential, suggesting the deposition of a Co(III)-based insoluble product on the electrode surface. The stability of the catalyst is also studied at several potentials in both buffered and nonbuffered electrolyte by monitoring the real-time mass variations.

Employing denaturation for rapid electrochemical detection of myoglobin using TiO2 nanotubes

An alternative antibody-free strategy for the rapid electrochemical detection of cardiac myoglobin has been demonstrated here using hydrothermally synthesized TiO2 nanotubes (Ti-NT). The denaturant induced unfolding of myoglobin led to easy access of the deeply buried electroactive heme center and thus the efficient reversible electron transfer from protein to electrode surface. The sensing performance of the Ti-NT modified electrodes were compared vis a vis commercially available titania and GCEs. The tubular morphology of the Ti-NT led to facile transfer of electrons to the electrode surface, which eventually provided a linear current response (obtained from cyclic voltammetry) over a wide range of Mb concentration. The sensitivity of the Ti-NT based sensor was remarkable and was equal to 18 mu A mg(-1) ml (detection limit = 50 nM). This coupled with the rapid analysis time of a few tens of minutes (compared to a few days for ELISA) demonstrates its potential usefulness for the early detection of acute myocardial infarction (AMI).

Multiplexed DNA-mediated electrochemistry

The aromatic core of double helical DNA possesses the unique and remarkable ability to form a conduit for electrons to travel over exceptionally long molecular distances. This core of π-stacked nucleobases creates an efficient pathway for charge transfer to proceed that is exquisitely sensitive to even subtle perturbations. Ground state electrochemistry of DNA-modified electrodes has been one of the major techniques used both to investigate and to harness the property of DNA-mediated charge transfer. DNA-modified electrodes have been an essential tool for both gaining insights into the fundamental properties of DNA and, due to the exquisite specificity of DNA-mediated charge transfer for the integrity of the π-stack, for use in next generation diagnostic sensing. Here, multiplexed DNA-modified electrodes are used to (i) gain new insights on the electrochemical coupling of metalloproteins to the DNA π-stack with relevance to the fundaments of in vivo DNA-mediated charge transfer and (ii) enhance the overall sensitivity of DNA-mediated reduction for use in the detection of low abundance diagnostic targets.

First, Methylene Blue (MB′) was covalently attached to DNA through a flexible C12 alkyl linker to yield a new redox reporter for DNA electrochemistry measurements with enhanced sensitivity. Tethered, intercalated MB′ was reduced through DNA-mediated charge transport. The redox signal intensity for MB′-dT-C12-DNA was found to be at least 3 fold larger than that of previously used Nile Blue (NB)-dT-DNA, which is coupled to the base stack via direct conjugation. The signal attenuation, due to an intervening mismatch, and therefore the degree of DNA-mediated reduction, does, however, depend on the DNA film morphology and the backfilling agent used to passivate the surface. These results highlight two possible mechanisms for the reduction of MB′ on the DNA-modified electrode that are distinguishable by their kinetics: reduction mediated by the DNA base pair stack and direct surface reduction of MB′ at the electrode. The extent of direct reduction at the surface can be minimized by overall DNA assembly conditions.

Next, a series of intercalation-based DNA-mediated electrochemical reporters were developed, using a flexible alkane linkage to validate and explore their DNA-mediated reduction. The general mechanism for the reduction of distally bound redox active species, covalently tethered to DNA through flexible alkyl linkages, was established to be an intraduplex DNA-mediated pathway. MB, NB, and anthraquinone were covalently tethered to DNA with three different covalent linkages. The extent of electronic coupling of the reporter was shown to correlate with the DNA binding affinity of the redox active species, supporting an intercalative mechanism. These electrochemical signals were shown to be exceptionally sensitive to a single intervening π-stack perturbation, an AC mismatch, in a densely packed DNA monolayer, which further supports that the reduction is DNA-mediated. Finally, this DNA-mediated reduction of MB occurs primarily via intra- rather than inter duplex intercalation, as probed through varying the proximity and integrity of the neighboring duplex DNA. Further gains to electrochemical sensitivity of our DNA-modified devices were then achieved through the application of electrocatalytic signal amplification using these solvent accessible intercalative reporters, MB-dT-C8, and hemoglobin as a novel electron sink. Electrocatalysis offers an excellent means of electrochemical signal amplification, yet in DNA based sensors, its application has been limited due to strict assembly conditions. We describe the use of hemoglobin as a robust and effective electron sink for electrocatalysis in DNA sensing on low density DNA films. Protein shielding of the heme redox center minimizes direct reduction at the electrode surface and permits assays on low density DNA films. Electrocatalysis of MB that is covalently tethered to the DNA by a flexible alkyl linkage allows for efficient interactions with both the base stack and hemoglobin. Consistent suppression of the redox signal upon incorporation of single CA mismatch in the DNA oligomer demonstrates that both the unamplified and the electrocatalytically amplified redox signals are generated through DNA-mediated charge transport. Electrocatalysis with hemoglobin is robust: it is stable to pH and temperature variations. The utility and applicability of electrocatalysis with hemoglobin is demonstrated through restriction enzyme detection, and an enhancement in sensitivity permits femtomole DNA sampling.

Finally, we expanded the application of our multiplexed DNA-modified electrodes to the electrochemical characterization of DNA-bound proteins containing [4Fe-4S] clusters. DNA-modified electrodes have become an essential tool for the characterization of the redox chemistry of DNA repair proteins that contain redox cofactors. Multiplexed analysis of EndonucleaseIII (EndoIII), a DNA repair protein containing a [4Fe-4S] cluster known to be accessible via DNA-mediated charge transport, elucidated subtle differences in the electrochemical behavior as a function of DNA morphology. DNA-bound EndoIII is seen to have two different electron transfer pathways for reduction, either through the DNA base stack or through direct surface reduction. Closely packed DNA films, where the protein has limited surface accessibility, produce electrochemical signals reflecting electron transfer that is DNA-mediated. The electrochemical comparison of EndoIII mutants, including a new family of mutations altering the electrostatics surrounding the [4Fe-4S] cluster, was able to be quantitatively performed. While little change in the midpoint potential was found for this family of mutants, significant variations in the efficiency of DNA-mediated electron transfer were apparent. Based on the stability of these proteins, examined by circular dichroism, we propose that the electron transfer pathway can be perturbed not only by the removal of aromatic residues, but also through changes in solvation near the cluster.

DNA-Mediated Charge Transport Devices for Protein Detection

Detection of biologically relevant targets, including small molecules, proteins, DNA, and RNA, is vital for fundamental research as well as clinical diagnostics. Sensors with biological elements provide a natural foundation for such devices because of the inherent recognition capabilities of biomolecules. Electrochemical DNA platforms are simple, sensitive, and do not require complex target labeling or expensive instrumentation. Sensitivity and specificity are added to DNA electrochemical platforms when the physical properties of DNA are harnessed. The inherent structure of DNA, with its stacked core of aromatic bases, enables DNA to act as a wire via DNA-mediated charge transport (DNA CT). DNA CT is not only robust over long molecular distances of at least 34 nm, but is also especially sensitive to anything that perturbs proper base stacking, including DNA mismatches, lesions, or DNA-binding proteins that distort the π-stack. Electrochemical sensors based on DNA CT have previously been used for single-nucleotide polymorphism detection, hybridization assays, and DNA-binding protein detection. Here, improvements to (i) the structure of DNA monolayers and (ii) the signal amplification with DNA CT platforms for improved sensitivity and detection are described.

First, improvements to the control over DNA monolayer formation are reported through the incorporation of copper-free click chemistry into DNA monolayer assembly. As opposed to conventional film formation involving the self-assembly of thiolated DNA, copper-free click chemistry enables DNA to be tethered to a pre-formed mixed alkylthiol monolayer. The total amount of DNA in the final film is directly related to the amount of azide in the underlying alkylthiol monolayer. DNA monolayers formed with this technique are significantly more homogeneous and lower density, with a larger amount of individual helices exposed to the analyte solution. With these improved monolayers, significantly more sensitive detection of the transcription factor TATA binding protein (TBP) is achieved.

Using low-density DNA monolayers, two-electrode DNA arrays were designed and fabricated to enable the placement of multiple DNA sequences onto a single underlying electrode. To pattern DNA onto the primary electrode surface of these arrays, a copper precatalyst for click chemistry was electrochemically activated at the secondary electrode. The location of the secondary electrode relative to the primary electrode enabled the patterning of up to four sequences of DNA onto a single electrode surface. As opposed to conventional electrochemical readout from the primary, DNA-modified electrode, a secondary microelectrode, coupled with electrocatalytic signal amplification, enables more sensitive detection with spatial resolution on the DNA array electrode surface. Using this two-electrode platform, arrays have been formed that facilitate differentiation between well-matched and mismatched sequences, detection of transcription factors, and sequence-selective DNA hybridization, all with the incorporation of internal controls.

For effective clinical detection, the two working electrode platform was multiplexed to contain two complementary arrays, each with fifteen electrodes. This platform, coupled with low density DNA monolayers and electrocatalysis with readout from a secondary electrode, enabled even more sensitive detection from especially small volumes (4 μL per well). This multiplexed platform has enabled the simultaneous detection of two transcription factors, TBP and CopG, with surface dissociation constants comparable to their solution dissociation constants.

With the sensitivity and selectivity obtained from the multiplexed, two working electrode array, an electrochemical signal-on assay for activity of the human methyltransferase DNMT1 was incorporated. DNMT1 is the most abundant human methyltransferase, and its aberrant methylation has been linked to the development of cancer. However, current methods to monitor methyltransferase activity are either ineffective with crude samples or are impractical to develop for clinical applications due to a reliance on radioactivity. Electrochemical detection of methyltransferase activity, in contrast, circumvents these issues. The signal-on detection assay translates methylation events into electrochemical signals via a methylation-specific restriction enzyme. Using the two working electrode platform combined with this assay, DNMT1 activity from tumor and healthy adjacent tissue lysate were evaluated. Our electrochemical measurements revealed significant differences in methyltransferase activity between tumor tissue and healthy adjacent tissue.

As differential activity was observed between colorectal tumor tissue and healthy adjacent tissue, ten tumor sets were subsequently analyzed for DNMT1 activity both electrochemically and by tritium incorporation. These results were compared to expression levels of DNMT1, measured by qPCR, and total DNMT1 protein content, measured by Western blot. The only trend detected was that hyperactivity was observed in the tumor samples as compared to the healthy adjacent tissue when measured electrochemically. These advances in DNA CT-based platforms have propelled this class of sensors from the purely academic realm into the realm of clinically relevant detection.

Estudo do crescimento bacteriano e da aplicação de procedimentos de limpeza e desinfecção no aço inoxidável 304L

Técnicas de limpeza química ou mecânica são comumente empregadas para evitar a formação de biofilmes e problemas associados com a colonização de superfícies por micro-organismos. Neste trabalho, amostras de aço inox 304L foram submetidas a ensaios acelerados de crescimento de biofilme, o qual foi posteriormente removido por tratamentos de limpeza e desinfecção (choque térmico com água a 70C por 1h, limpeza com ácido fosfórico 20 % v/v por 30min e desinfecção com peróxido de hidrogênio 0,17 % v/v por 1h). Com o objetivo de verificar a influência destes tratamentos na remoção do biofilme, este foi caracterizado (por quantificação microbiana e análises de espectroscopia de impedância eletroquímica - EIE), antes e após a aplicação dos tratamentos. Dois casos foram estudados. No primeiro caso, simularam-se condições presentes em tubulações de ambientes hospitalares e sistemas de distribuição de água quente. O micro-organismo utilizado foi a bactéria Serratia marcences. O processo de formação de biofilme e posterior limpeza e desinfecção foi realizado de modo contínuo durante 15 semanas. Os resultados de quantificação microbiana e de EIE mostraram que desde a primeira semana de exposição e ao longo dos ensaios, formou-se um biofilme aderente à superfície do aço e que o emprego dos tratamentos de limpeza e desinfecção não foi eficaz na remoção do biofilme. Em um segundo caso, simularam-se condições presentes em tubulações da indústria de água mineral, empregando-se a bactéria Pseudomonas aeruginosa. Neste caso, as técnicas de limpeza e desinfecção foram aplicadas individualmente e em conjunto. A aplicação de choque térmico, assim como da limpeza ácida, sobre um biofilme com 72 h de formação foi capaz de eliminar as bactérias viáveis, presentes na superfície do aço. Entretanto, a desinfecção com peróxido de hidrogênio não foi capaz de eliminá-las. Nas duas condições, porém, as análises de EIE do sistema aço/biofilme mostraram que o mesmo não foi completamente removido da superfície do metal. Correlacionando os dois casos, pode-se inferir que a superfície do aço inox 304L é rapidamente colonizada pelas duas espécies microbianas e que as técnicas de limpeza e desinfecção são capazes de reduzir e até eliminar as células viáveis, embora não removam completamente o biofilme da superfície. Por outro lado, é importante ressaltar que fatores como a arquitetura, espessura e porosidade do biofilme, e propriedades intrínsecas da superfície, são fatores que afetam os valores de impedância medidos, sendo necessário considerá-los na análise, tanto da formação do biofilme, quanto dos efeitos dos procedimentos de limpeza e desinfecção

Orthorhombic α-moo 3 coatings with lath-shaped morphology developed by SPPS: Applications to super-capacitors

Electrical double-layer capacitors owe their large capacitance to the formation of a double-layer at the electrode/electrolyte interface of high surface area carbon-based electrode materials. Greater electrical energy storage capacity has been attributed to transition metal oxides/nitrides that undergo fast, reversible redox reactions at the electrode surface (pseudo-capacitive behavior) in addition to forming electrical double-layers. Solution Precursor Plasma Spray (SPPS) has shown promise for depositing porous, high surface area transition metal oxides. This investigation explored the potential of SPPS to fabricate a-MoO 3 coatings with micro-structures suitable for use as super-capacitor electrodes. The effects of number of spray passes, spray distance, solution concentration, flow rate and spray velocity on the chemistry and micro-structure of the a-MoO 3 deposits were examined. DTA/TGA, SEM, XRD, and electrochemical analyses were performed to characterize the coatings. The results demonstrate the importance of post-deposition heating of the deposit by subsequent passes of the plasma on the coating morphology. © ASM International.

Studying adsorbent dynamics on a quartz crystal resonator using its nonlinear electrical response

The quartz crystal resonator has been traditionally employed in studying surface-confined physisorbed films and particles by measuring dissipation and frequency shifts. However, theoretical interpretation of the experimental observations is often challenged due to limited understanding of physical interaction mechanisms at the interfaces involved. Here we model a physisorbed interaction between particles and gold electrode surface of a quartz crystal and demonstrate how the nonlinear modulation of the electric response of the crystal due to the nonlinear interaction forces may be used to study the dynamics of the particles. In particular, we show that the graphs of the deviation in the third Fourier harmonic response versus oscillation amplitude provide important information about the onset, progress and nature of sliding of the particles. The graphs also present a signature of the surface-particle interaction and could be used to estimate the interaction energy profile. Interestingly, the insights gained from the model help to explain some of the experimental observations with physisorbed streptavidin-coated polystyrene microbeads on quartz resonators. © 2012 Elsevier B.V. All rights reserved.

Optimizing the energy offset between dye and hole-transporting material in solid-state dye-sensitized solar cells

The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenylamino)-9,9′- spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials. © 2013 American Chemical Society.