Journal of Alloys and Compounds 747 (2018) 149e155
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Design complexity of DPN patterning with Cr3þ and Co2þ metallic ions on Au (111) thin film , Daniel Marconi* Adrian Calborean, Ioana Grosu, Alia Colnit¸a Department of Molecular and Biomolecular Physics, National Institute for Research and Development of Isotopic and Molecular Technologies, Donat 67-103, 400293, Cluj-Napoca, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2017 Received in revised form 27 February 2018 Accepted 1 March 2018 Available online 3 March 2018
We present in this work a fabrication chain of an electrochemical sensor platform based on detection of Cr3þ and Co2þ metallic ions species. Through molecular beam epitaxy (MBE) technique, a high quality Au (111) thin film was firstly fabricated in ultra-high vacuum (UHV) conditions, and used as support for chemical SAMs functionalization with 1,4-dithiothreitol (DTT) spacers. On the formed hybrid surface/ linker structures, we employed Dip Pen Nanolithography (DPN) for patterning various templates of Cr3þ and Co2þ metallic ion species. Coated AFM tips with metallic molecular inks were used to fabricate different paths on the Au substrate, offering a unique functional design complexity. By varying the tip speed and/or dwell time, micro-nano-scale templates of both species were patterned in the form of square, double-square, triangle and lines. A full control of molecular ink transport mechanisms during the drawing process was assured, succeeding the writing of Cr3þ and Co2þ metallic ions on Au film substrate. Surface topology analyzed by Lateral Force Microscopy demonstrated the chemical contrast of patterned species, being a clear indicative of DPN tracing paths. Molecular recognition of both Cr3þ and Co2þ metallic ions has been made by potentio-electrochemical impedance spectroscopy (PEIS) measurements. The Nyquist plots were compared and the equivalent circuits obtained in both conformations (Cr3þ/DTT/Au, Co2þ/DTT/Au) were discussed in the light of experimental parameters. © 2018 Elsevier B.V. All rights reserved.
Keywords: Electrochemical sensor platform Au(111) thin film fabrication DPN patterning Electrochemical impedance spectroscopy
1. Introduction As a continuous tailoring of current electronic architectures [1e3], nanoparticle devices incorporating transition elements of dblock were intensively investigated in the last years due to their tremendous potential in various technological applications. Their scientific importance, in particular in the fields of photovoltaics [4], bio-nanotechnology [5] or integrated molecular electronics [6], was already demonstrated and certified as an emerging route to further miniaturization. Beside the possible response to nano-electronics of the future, molecular electronics allows to assemble a large number of nanoscale structures on the same substrate, which finnaly lead to the fabrication of new devices and/or circuit architectures. As such, the main goal is focused nowadays to the development of new technologies that incorporate molecular e scale elements. Towards molecular electronics, biological and biomedical
* Corresponding author. E-mail address:
[email protected] (D. Marconi). https://doi.org/10.1016/j.jallcom.2018.03.009 0925-8388/© 2018 Elsevier B.V. All rights reserved.
applications implying SAM functionalization [7] of molecular systems are of great interest, in particular for the development of sensor platforms based on molecular recognition engineering [8e10]. The final aim is to provide both switching and sensing characteristics at single particle scale [11]. Moreover, the possibility to assembly a large palette of nanoscale subjects will open new paths to circuit architectures and electronic devices [12e15]. The drawback in all cases, whatever the application, arrives from the method used for the fabrication at reduced scale [16,17]. The technological limits are drastically affected depending on the chosen substrate, linker, or the anchored molecular species. If we strictly refer to the design of a powerful miniaturized sensing platform, which is the case of this work, then the substrate quality and the fabrication conditions are very important in order to obtain, a defect-free Au thin film. A clean environment, such as ultra-high vacuum (UHV) is vital to minimize the incorporation of unintentional impurities, avoiding to affect the surface morphology and the lithographic process. In this perspective, we present here a fabrication chain of a miniaturized electrochemical sensor platform for detection of Cr3þ
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and Co2þ metallic ions, from the Au(111) thin film manufacturing, chemical SAMs functionalization, to Dip Pen Nanolithography (DPN) ink patterning of metallic ion species. Molecular beam epitaxy (MBE) deposition technique was firstly employed to fabricate a high quality Au(111) thin film. By using UHV conditions we have minimized the incorporation of unintentional impurities, leading to large atomically flat terraces. The deposited Au film was used as working electrode in the electrochemical impedance analysis. In a second step, a chemical protocol for SAMs functionalization of 1,4-dithiothreitol (DTT) has been developed. The formed hybrid surface/linker structures were then employed to attach by DPN approach [18] the Cr3þ and Co2þ molecular species. DTT linker, adsorbed on Au substrate, has been previously used by the group of Mandler et al. [19] to investigate the heterogeneous binding of Mg2þ, Ca2þ and Sr2þ metal ions. In the same register, DTT- modified Au thin films were reported by Magura et al. [20], while the group of Vela [21] used SAMs of DTT on Au surface for phospholipid bilayer formation. However, our particular feature in comparison with the above mentioned investigations is the use of high resolution DPN technique, which allowed us to deposit the Cr3þ and Co2þ metallic ions on the same fabricated Au thin film, offering a unique functional design complexity. Through the coated AFM tip, we were able to directly place at a defined location the Cr3þ and Co2þ species on the previous functionalized Au thin film. The capillary forces enabled a sustained and perpetual flow of both molecular inks, while the attraction and repulsion forces which appeared at interface solute/ substrate has afforded the self e assembly processes [22]. During the patterning process, the working parameters of AFM tip were carefully controlled in order to establish the high resolution of the Au film and even of the lithographic protocol itself. Tip immersion time, writing speed or scanning speed, corroborated with the diffusion characteristics of novel formatted molecular inks were critical parameters that assured the DPN patterning process. The topological characterization of fabricated Au thin film was firstly made by Scanning Tunnelling Microscopy (STM) in UHV environment, without contaminating the surface. After DPN depositions, high-performance AFM imaging was employed after each DPN pattern deposition. Finnaly, a molecular recognition analysis was performed through impedance spectroscopy measurements [23]. Potentiostatic Electrochemical Impedance Spectroscopy (PEIS) method was used in particular, emphasising the benefits of using the electrochemical approaches in determination of biological and environmental analysis [24e30]. The Nyquist plots were compared and the equivalent circuits were obtained in both conformations (Cr3þ/DTT/ Au, Co2þ/DTT/Au). 2. Materials and methods 2.1. Au (111) thin film formation The metallic substrate used in this experiment was a Au/Si(111) thin film deposited in UHV environment (~1010 mbar) by MBE technique using a Lab - 10 MBE, Omicron GmbH system. A 7 7 reconstructed Si(111) (p-type, resistivity 0.01 U cm) substrate was obtained through a well-established thermal treatment [31]. The quality of the reconstructed Si(111) surface was demonstrated in the early works of our group [32,33]. Gold pellets (99.9995% purity, Premion, Alfa, Germany) were evaporated from an effusion cell on a substrate heated at 580 C, with an evaporation rate of 1.6 nm/min and a deposition time of 1 h. The film deposition was followed by a 1 h annealing treatment at the deposition temperature of 580 C and a controlled decrease of the substrate temperature with 13 C/
min, until it reached room temperature. The thickness of the film was 95 nm and determined by using a beam flux monitor attached to the deposition manipulator. Scanning tunnelling microscopy (STM) images of the Au film were obtained using an UHV, variable temperature STM (VT-STM by Omicron GmbH, Germany) and analyzed through SPIP (Scanning Probe Image Processor e SPIP, version 6.0.10, Image Metrology ApS, Lyngby, Denmark) software package. 2.2. Self-assembled monolayers (SAMs) of 1,4-dithiothreitol(DTT) (i) The DTT compound has been achieved as powder from Sigma Aldrich and used without any complementary purification procedures. It was prepared as ethanol solution following the concentration of 0.5 mg/mL. The resulting mixture was stirred magnetically for 10 min. Immediately after formation, the solution was light screened and deposited at a temperature of 4 C. ii) ( A carefully cleaning procedure of Au substrate was performed, being repeatedly washed with ethanol and deionized water (Millipore Gradient) and dried under nitrogen flow. Then, it was immersed in the above obtained ethanol solution of DTT for 24 h at room temperature. After extraction from solution, the substrate was washed again following the same protocol.
2.3. Molecular inks Two solutions containing Cr3þ and Co2þ metallic ions were prepared as molecular inks for DPN patterning. Cr(NO3)39H2O (0.48 g) and CoCl2$4H2O (0.35 g) compounds were firstly weighed, then 2 ml of deionized water were added, obtaining a concentration of 0.6 M for the both solutions. The formed solutions were stirred magnetically 10 min and immediately used for DPN molecular attachment to DTT - SAMs linkers previously attached. 2.4. Dip Pen Nanolithography DPN 5000 equipment of NanoInk USA, has been used for micronano-lithography templates. The system was protected by a Echamber glove box connected to a humidity NanoScriptor controller that was set to relative humidity (RH) of 53%. The experiments were performed at room temperature of 23 C. High resolution contact golden silicon cantilevers, namely CSG30 series, were purchased from NT-MDT and used as AFM e dip pen writer. Their compatible specifications (cantilever - length 190 mm, width 30 mm, thickness 1.5 mm, resonant frequency in the range 24e76 kHz, force constant in the range 0.13e2 N/m) with DPN technology assured both functionality and control during the patterning process. Contact mode operation was employed, while for the surface topological imaging we used Lateral Force Microscopy (LFM) technique. As a preparation procedure for writing process, the AFM tips were initially cleaned in piranha solution, rinsed several times with deionized water and dried under nitrogen stream. After removal, they were fully immersed in molecular solutions for 60 s, and then gently dried under nitrogen. Immediately, the probes were mounted in the clip holders and installed on the DPN instrument. 2.5. Potentio electrochemical impedance spectroscopy (PEIS) We used Potentiostatic Electrochemical Impedance Spectroscopy method, which basically apply a constant potential E and perform impedance through potentiostatic mode to a frequency
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domain. Spectroscopic impedance measurements were made by a VSP multi-channel potentiostat manufactured by BioLogic Science Instruments, coupled with a 5 A external booster. An AC modulation voltage of 10 mV was fixed and the frequency domain was chosen between 1 MHz and 100 mHz. A standard electrode connection has been used, as shown in Scheme 1. In such design, the current crosses along the cell from CA2 to CA1, while the potential is measured between Ref1 and Ref2. The working electrode is connected to REF1þCA2, the counter eelectrode is connected to REF3þCA1, while the reference electrode is connected to REF2. All the PEIS measurements were performed in deionized water electrolyte. The measurements were performed in three phases. As a first step, we tested the formed Au film without any DTT functionalization. In the second step, we used the functionalized Au film, and in the final step, we compared the impedance signal of the Au film after each DPN patterning of Cr3þ and Co2þ metallic ions. An equivalent circuit was built with passive elements, allowing a very good impedance fitting, thus assigning a reliable physical meaning to experimental data.
3. Results 3.1. Au (111) thin film fabrication Au (111) thin film fabrication was made entirely in house by MBE. Immediately after deposition, the incorporated VT-STM equipment was used for the morphological analysis of Au thin film. The STM images (Fig. 1a and b) of the deposited film, shows a clearly 2D layer by layer growth trend on large areas of the surface, indicating that the 580 C substrate temperature is quite suitable for the formation of atomic terraces with small roughness. From the 1 mm 1 mm 3D STM image (Fig. 1a) one could observe large terraces, separated by atomic steps on the surface of the film. Moreover, the Au nano-islands easily observed in the 500 nm 500 nm 3D STM image (Fig. 1b) could indicate the existence of nucleation centres along the surface. The chemical, thermodynamic and physical mechanisms that determine the growth of the Au(111) thin film were appropriate for obtaining a high purity and crystal quality of the surface. The fabricated substrate was adequate for SAMs functionalization with DTT spacers and subsequently for attachment of metallic ion species. For electrochemical sensor platform, we used the substrate as Au electrode for electrochemical impedance measurements, playing the role of sensor support for Cr3þ and Co2þ detection.
Scheme 1. PEIS electrode connection.
Fig. 1. a) 1 mm 1 mm and b) 500 nm 500 nm 3D STM images of the deposited Au layer. The images were recorded by a bias potential of 2 V and a tunnelling current of 0.3 nA.
3.2. DPN micro/nanopatterning DPN patterning was performed on the DTT SAMs/Au support. The two metallic molecular inks containing Cr3þ and Co2þ ions were used separately to trace different paths on the functionalized Au substrate. A schematic procedure integrating the DPN writing process through solvent meniscus, in particular of ion species gearing on the DTT SAMs previously attached on Au surface by chemical procedures is highlighted in Fig. 2. We used a similar approach in a previous work [34]. Using contact mode approach for the DPN writing, we fabricated specific structures (square, double-square, triangle, lines) at micro and nano-scale. The deionized water was changed after each procedure in order to avoid any spurious mixing of the two molecular species. For each molecular ink we have chosen different places on the Au thin film, having a full control of the drawing process. A selective placement, at specific sites and with custom templates, was performed. For surface topography we used the Lateral Force Microscopy (LFM) mode, thus the chemical contrast information offered in this case was a clear indicative of DPN writing within the
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Fig. 2. Schematic design from isolated DTT solution to DTT -SAM formation and DPN transport mechanism of the metallic ions.
two species. Fig. 3 describes a square path of Cr3þ ions on Au together with an averaged horizontal cross section dedicated to Z height analysis. A smaller 3D LFM image was added in the cross section graph, highlighting the patterned area. As can be seen, a thickness around 8 nm was obtained on Z direction. A similar square path was fabricated for Co2þ metallic ions, obtaining a similar result. A small gap difference was seen on the thickness of deposited Co metallic ions, with a value of ~6 nm in Z height. In another attempt, we succeeded to pattern a Co2þ structure with 2 mm 1 mm dimensions (see Fig. 4). It should be mentioned that for the first square paths of Cr3þ and 2þ Co , we cleaned the Au surface after each DPN drawing, and then we performed PEIS impedance spectroscopy measurements, as will be described in the following section. After the fabrication of square structures, we patterned with Co2þ molecular ink an isosceles triangle shape on the DTT/Au thin film. In Fig. 5 is shown the AFM topography of such pattern. In the last conducted experiment, we decreased the fabrication scale to nano-dimension, thus we succeeded to fabricate path lines of Cr3þ metallic ions. In Fig. 6 is shown the LFM image of patterned lines, where different thicknesses of lines between 223 nm and 111 nm were obtained. The averaged cross section after DPN writing revealed a height of around 6 nm. It should be reminded that in the preliminary tests, we tried without any success to write directly with the Cr3þ and Co2þ molecular ink solutions, without DTT spacers on Au. As expected, no paths were drawn. Bottom e up methodology demonstrated here, allowed us to further imagine this architecture as a sensing platform which can discern between different metallic ions, but also can differentiate on structure shapes or dimension.
3.3. Impedance PEIS measurements We performed PEIS measurements at different moments of the fabrication process. Firstly, the impedance measurements were made on the isolated Au thin film without any functionalization, in deionized water. After SAMs functionalization with DTT linkers, we made a new PEIS analysis. In a final step, we performed investigations after each lithographic process with Cr3þ and Co2þ (we made paths 1 mm 1 mm for both) metallic ions. The PEIS measurements were made in this case at the same level, thus avoiding any electrical signal overestimation. The response in frequency for all systems was represented by Nyquist plots. As can be seen in Fig. 7, a clear difference in impedance amplitude was obtained from isolated Au thin film to DTT/Au assembly or between the inscriptioned Cr3þ and Co2þ metallic ions. As expected, this dissimilarity arrives from the physicochemical processes depending on frequency which occurred in each case. Distinct systems vibrations will result, thus the electric signal in impedance is different from case to case. If we focused on the behaviour of the two Cr3þ and CO2þ metallic ions, a small shifting for CO2þ species appeared. A possible source of such change can be attributed to the metal linker rapport in respect to the Au thin film. Ionic radius is different for Cr (0.58) and Co (0.7), thus will influence the coordination number of each complex. Moreover, the size of the metallic ion will display a different charge, and in consequence a different electronic configuration. All these factors could influence the electrical behaviour, yielding the shifting in impedance between the Cr3þ and Co2þ metallic ions. Nevertheless, an interpretation of impedance electrical
Fig. 3. 2D LFM image of Cr path - 1 mm 1 mm square (left) and horizontal cross section with a 3D image in the square zone.
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Fig. 4. 2D LFM image of Co path - 2 mm 2 mm square (left) and horizontal cross section with a 3D image in the square zone.
Fig. 5. 2D LFM image of Co isosceles triangle shape (left) and horizontal cross section with a 3D image inside (right).
Fig. 6. 2D LFM image of Cr path lines (left) and horizontal cross section (right) with a 3D image.
behaviour demands a suitable circuit model for estimating the experimental parameters or behaviour prediction under different conditions. An ideal equivalent circuit which fit very well our systems is represented in Fig. 8, in which C1 represents the electrochemical double layer capacity, R1 is the uncompensated ohmic resistance and Q1 is the constant phase element e an imperfect capacitor that behaves as a double layer. The latter element can be assigned to the partition of physical properties in Au thin films, in
the way normal to the electrode area (interfacial capacitance). We chose this equivalent circuit, taking into account the capacitive dispersion that appears in solid electrodes which results from electrode surface irregularities. This will lead to a dependency on phase-angle frequency which is basically described by a constant phase element (CPE). As can be observed in Figs. 9 and 10, the impedance fitting is in very good agreement with the experimental data.
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Fig. 7. Nyquist diagram of: Blue e Au thin film; Red e DTT SAMs on Au; Green e Cr ions on DTT/Au thin film; Black e Co ions on DTT/Au thin film. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
processes depending on each system, we added in Table 1, the circuit values corresponding to the two species. As can be observed, the main difference occurs on R1 charge transfer resistance, with a significant decrease for Co2þ species. One plausible explanation for the loss of resistance is given by the different molecular interactions that occur between neighbouring DTT spacers on the Au film. The lowest thickness obtained for Co/ DTT/Au structure can influence the diffusion of electric field along the Au electrode, thus resulting in a different compositional tailoring effect from Cr3þ to Co2þ species. Diffusion related to electric resistance modifications depending on metallic ion demonstrate an inhibition of electron transfer at DTT/Au interface. Cr3þ and Co2þ ionic resistive features of the functionalized SAMs differentiate their impedance signal, allowing us to imagine the whole design as a platform sensor for heavy metals detection. 4. Conclusions
Fig. 8. Equivalent electrical circuit of Cr3þ and Co2þ molecular species attached on DTT/Au thin film.
Fig. 9. Z fit of Cr3þ species on DTT/Au assembly.
Fig. 10. Z fit of Co2þ species on DTT/Au assembly.
Using MBE fabrication technique, we fabricated a high quality Au(111) thin film in UHV conditions with a gold thickness of 95 nm. Atomically flat terraces were obtained, allowing to DTT spacers to be functionalized as SAMs. The developed structures were lately used as support for patterning various forms of Cr3þ and Co2þ molecular inks, with the main of goal of building a sensor platform for detection of the two metallic ions. A first surface modification was performed by SAMs functionalization with DTT spacers. This intermediate step was made in order to assure the immobilization of metallic ions between the hydroxyl groups. DPN approach was employed for the writing with Cr3þ and Co2þ metallic ions, offering a unique advantage to a selective inscription of the molecular species on the same surface. Moreover, we created particular structures with both Cr3þ and 2þ Co , from 1 mm 1 mm squares, to 2 mm 1 mm squares or a triangle with sides of about 1 mm. At nanoscale dimension, we succeeded to pattern five lines with different thicknesses from 223 nm to 128 nm. The nature of chemical interaction between both molecular inks and the Au/DTT substrate was demonstrated. Au surface modification was controlled by DPN, demonstrating affinity assays of the two molecular species. Thus, all fabricated paths described here can serve as further sensing devices or used in another integrated molecular technologies creating flexible connections and orientations of/between materials. This sentence is based on the manner of manipulating and positioning the molecular species at micro-nano length scale. The PEIS measurements emphasized the electrical behaviour of Au thin film (used as contact electrode), of DTT SAMs on Au and of the two metallic ions patterned on the surface. The detection of Cr3þ and Co2þ metallic ions grafted on Au substrate was monitored as an impedance shifting. The Nyquist diagrams revealed a signal difference between the two systems, thus yielding particular metal ion recognition domains. The differences in performance of electrical impedance were assessed by an equivalent circuit model which fit very well the Nyquist plots, being able to nearly estimate the experimental parameters. Acknowledgments
Table 1 Comparison of the values using the Z fit equivalent circuit. Equivalent circuit
C1(F)
R1(Ohm)
Q1(F.s^(a1)
a1
Cr3þ/DTT/Au Co2þ/DTT/Au
27.05 e -12 28.32 e -12
125329 97241
6.061 e -6 7.436 e -6
0.743 0.703
Financial support from the National Authority for Scientific Research and Innovation e ANCSI, Core Programme, Project PN1630 01 02 is acknowledged. References
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