Antibiotic pollution of freshwaters and even food products has become an important concern worldwide. Hence, it is of utmost importance to develop cost-effective and reliable devices that can provide information on the presence of such contaminants to the general population. In the present work, zinc oxide (ZnO) nanotetrapods (NTP) produced via a high yield laser processing approach were used as transducers in a luminescent-based immunosensor to detect tetracycline (TC). These tetrapodal structures present needle-shaped branches with a high aspect ratio, exhibiting lengths from hundreds of nanometers to a few micrometers and an average thickness of tens of nanometers, providing a high surface area for bioreceptor immobilization and analyte reaction, which is quite desirable in a transducer material. Besides, these ZnO NTP display intense photoluminescence (PL) at room temperature, making such a signal rather promising for transduction. Indeed, the intensity of the ZnO PL signal was seen to correlate with the TC concentration. The PL quenching with increasing analyte concentration is explained considering the rise in the bending of the electronic bands of the semiconductor near its surface due to increased charge density at this region, induced by the interaction between the bioreceptor (anti-TC antibodies) and the TC molecules. As a larger depletion width (and potential barrier) is promoted near the surface, the excitonic recombination probability is reduced and, consequently, the PL intensity in the ultraviolet spectral region, allowing us to use this relationship as a sensing mechanism. This information enabled us to define a calibration curve for TC quantification in the 0.001 to 1 μg L^–1 range, which is the range of interest of this antibiotic in freshwaters. A limit of detection (LOD) of ∼1.2 ng L^–1 is reported, corresponding to one of the lowest LOD found in the literature for this antibiotic, indicating that the present ZnO NTP-based biosensors rival the current state-of-the-art ones.

Published @DOI: 10.1021/acsanm.1c03838. ACS Applied Nano Materials, 2022

Zinc oxide (ZnO) nanostructures have been widely used in biosensor applications. However, little attention has been given to the interaction of ZnO structures with physiological buffer solutions. In the present work, it is shown that the use of buffers containing phosphate ions leads to the modification of the ZnO tetrapodal micro/nanostructures when immersed in such solutions for several hours, even at the physiological pH (7.4). ZnO samples designed to be used as transducers in biosensors were immersed in phosphate buffers for several durations at pH = 5.8 and pH = 7.4. Their detailed morphological, structural and optical characterization was carried out to demonstrate the effect of the ZnO interaction with the phosphate ions. The pH had an important role in the ZnO conversion into zinc phosphate, with lower pH promoting a more pronounced effect. After 72 h and at pH = 5.8, a significant amount of the ZnO structures were converted into crystalline zinc phosphate, while immersion during the same time at pH = 7.4 resulted predominantly in amorphous zinc phosphate particles mixed with the original ZnO tetrapods. Photoluminescence spectra show remarkable changes with prolonged immersion times, particularly when the luminescence of the sample was investigated at 14 K. These findings highlight the importance of a careful analysis of the sensing results when phosphate-based buffer solutions are in contact with the ZnO transducers, as the changes observed on the transduction signal during sensing experiments may also comprise a non-negligible contribution from a phosphate-induced transformation of ZnO, which can hamper an accurate assessment of the sensing behavior.

Published @DOI: 10.1016/j.mtchem.2021.100629. Materials Today Chemistry, 2021, 23, 100629

The inherent scalability, low production cost and mechanical flexibility of laser-induced graphene (LIG) combined with its high electrical conductivity, hierarchical porosity and large surface area are appealing characteristics for many applications. Still, other materials can be combined with LIG to provide added functionalities and enhanced performance. This work exploits the most adequate electrodeposition parameters to produce LIG/ZnO nanocomposites. Low-temperature pulsed electrodeposition allowed the conformal and controlled deposition of ZnO rods deep inside the LIG pores whilst maintaining its inherent porosity, which constitute fundamental advances regarding other methods for LIG/ZnO composite production. Compared to bare LIG, the composites more than doubled electrode capacitance up to 1.41 mF cm⁻² in 1 M KCl, while maintaining long-term cycle stability, low ohmic losses and swift electron transfer. The composites also display a luminescence band peaked at the orange/red spectral region, with the main excitation maxima at ~ 3.33 eV matching the expected for the ZnO bandgap at room temperature. A pronounced sub-bandgap tail of states with an onset absorption near 3.07 eV indicates a high amount of defect states, namely surface-related defects. This work shows that these environmentally sustainable multifunctional nanocomposites are valid alternatives for supercapacitors, electrochemical/optical biosensors and photocatalytic/photoelectrochemical devices.

Published @DOI:10.1038/s41598-021-96305-8. Scientific Reports 2021, 11, 17154.

Zinc oxide (ZnO)/laser-induced graphene (LIG) composites were prepared by mixing ZnO, grown by laser-assisted flow deposition, with LIG produced by laser irradiation of a polyimide, both in ambient conditions. Different ZnO:LIG ratios were used to infer the effect of this combination on the overall composite behavior. The optical properties, assessed by photoluminescence (PL), showed an intensity increase of the excitonic-related recombination with increasing LIG amounts, along with a reduction in the visible emission band. Charge-transfer processes between the two materials are proposed to justify these variations. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy evidenced increased electron transfer kinetics and an electrochemically active area with the amount of LIG incorporated in the composites. As the composites were designed to be used as transducer platforms in biosensing devices, their ability to detect and quantify hydrogen peroxide (H₂O₂) was assessed by both PL and CV analysis. The results demonstrated that both methods can be employed for sensing, displaying slightly distinct operation ranges that allow extending the detection range by combining both transduction approaches. Moreover, limits of detection as low as 0.11 mM were calculated in a tested concentration range from 0.8 to 32.7 mM, in line with the values required for their potential application in biosensors.

Published @DOI: 10.3390/chemosensors9050102. Chemosensors, 2021, 9 (5), 102

Metal oxide semiconductors, and particularly zinc oxide (ZnO), are amongst the most investigated materials to be applied as transducer elements in biosensing devices, due to properties as biochemical stability, biocompatibility and functionalisation simplicity. In the present work, ZnO nanotetrapods were produced by laser-assisted flow deposition to be used as transducer of a photoluminescence (PL)-based immunosensor to detect the human chorionic gonadotropin (hCG). The produced nanostructures were analysed by electron microscopy, Raman and PL spectroscopies. The ZnO tetrapods exhibit an intense PL in both the ultraviolet and visible range, which intensity changed after the direct immobilisation of the anti-hCG antibodies into the semiconductor’s surface. The changes in the PL intensity after the interaction of the immobilised antibodies with hCG were used as the transduction mechanism for the envisaged sensors. It was observed that when antibody-antigen interactions occur, charge rearrangement at the surface of the semiconductor leads to a reduction in overall intensity of the PL signal with increasing hCG concentration. This change was seen to be correlated to the ZnO-based transducers to be incorporated into optical biosensors. Additionally, the possible interaction mechanisms between the ZnO surface and the immobilised molecules responsible for the changes in the PL spectra were discussed.

Published @DOI: 10.1016/j.apsusc.2020.146813 Applied Surface Science, 2020, 527, 146813

A scalable laser scribing approach to produce zinc oxide (ZnO) decorated laser-induced graphene (LIG) in a unique laser-processing step was developed by irradiating a polyimide sheet covered with a Zn/ZnO precursor with a CO2 laser (10.6 μm) under ambient conditions. The laser scribing parameters revealed a strong impact on the surface morphology of the formed LIG, on ZnO microparticles' formation and distribution, as well as on the physical properties of the fashioned composites. The ZnO microparticles were seen to be randomly distributed along the LIG surface, with the amount and dimensions depending on the used laser processing conditions. Besides the synthesis conditions, the use of different precursors also resulted in distinct ZnO growth's yields and morphologies. Raman spectroscopy revealed the existence of both wurtzite-ZnO and sp2 carbon in the majority of the produced samples. Broad emission bands in the visible range and the typical ZnO near band edge (NBE) emission were detected by photoluminescence studies. The spectral shape of the luminescence signal was seen to be extremely sensitive to the employed processing parameters and precursors, highlighting their influence on the composites' optical defect distribution. The sample produced from the ZnO-based precursor evidenced the highest luminescence signal, with a dominant NBE recombination. Electrochemical measurements pointed to the existence of charge transfer processes between LIG and the ZnO particles.

Published @ DOI: 10.1039/C8NA00391B (Paper) Nanoscale Adv., 2019,1, 3252-3268

Zinc oxide (ZnO) is a widely versatile semiconductor with major importance from the technological point of view, presenting the advantage of being grown by a large number of techniques and having one of the richest varieties of morphologies. Due to the special interest in this semiconductor, several approaches have been employed to control the production of its nanostructures. Herein, we report the development of a vapour-based method, designated as laser-assisted flow deposition (LAFD), capable of producing ZnO micro and nanocrystals with different morphologies and with a high crystalline and optical quality. This new process allows a high yield of ZnO production, showing great prospects for scalable applications. In the present work, we review in detail the main growth parameters and their relationship with the produced morphologies, in addition to their influence on the structural and optical properties. Furthermore, an assessment of the possible growth mechanisms that may be involved in this new method is reported. Some potential applications of the ZnO structures produced by LAFD were also evaluated, with focus on the photocatalysis and photovoltaic fields. Additionally, the possibility of synthesizing ZnO composite nanostructures as well as the growth of other metal oxides using this technique was explored.

Published @DOI: 10.1039/C8CE01773E (Highlight) CrystEngComm, 2019, 21, 1071-1090

Presented @ 17th International conference on Advanced Nanomaterials

Presenter: Joana Rodrigues

Authors: J. Zanoni, S. O. Pereira, N. F. Santos, G. Gaspar, A. J. S. Fernandes, A. F. Carvalho, T. Monteiro F. M. Costa, J. Rodrigues

The increasing interest in biosensing and early diseases’ diagnosis has led to remarkable efforts in finding suitable materials and sensing mechanisms to achieve high sensitivity and selectivity at low production cost. Among the most investigated materials, metal oxides like zinc oxide (ZnO) and carbon-based materials have been playing an important role in the field due to their versatility and unique properties, namely their biochemical stability, biocompatibility and functionalisation simplicity. As such, the synergetic combination of these materials should result in advanced functional properties, which can be tailored to the desired application. In this framework, we report the development of ZnO/laser-induced graphene (LIG) composites by direct laser writing in a single processing step. We aim at using these composites as transducer platforms in the fabrication of optical and electrochemical devices for detection of biomedical analytes.

In this approach, a polyimide polymer covered with a ZnO precursor is irradiated with a CO₂ infrared laser under ambient conditions, producing a graphene foam with ZnO crystals. The simultaneous production of both components is expected to lead to a strong materials interconnection. Other advantages of this method include the production of flexible samples, a prompt synthesis process, easily scalable and the ability to perform patterned designs by using a computer-assisted laser-scribing system. The produced composites were fully characterised by electron microscopy, Raman and photoluminescence (PL) spectroscopies, as well as electrochemical analysis. The combination of laser processing conditions chosen for each sample was seen to have a noteworthy influence in the composite’s properties. Furthermore, tests monitoring the PL outcome as a function of the concentration of the human chorionic gonadotropin (hCG) hormone suggested their promising application in the production of biosensors with clinical interest.

Presented @ E-MRS Fall Meeting 2019

Best Poster Award

Presenter: Joana Rodrigues

Authors: J. Rodrigues, J. Zanoni, G. Gaspar, A. J. S. Fernandes, A. F. Carvalho, T. Monteiro F. M. Costa

The formation of zinc oxide (ZnO)/carbon composites has been subject to intense research since the synergistic behavior between these two materials is expected to result in advanced functional properties, useful in a wide range of applications, including photonics, optoelectronics, and general sensing.

A common feature in most of the ZnO/carbon composites is their multi-step preparation. Typically, both components are produced separately, with the desired dimensions and morphologies, and then mixed together to form the final composite material. In order to overcome the usual elaborated and time-consuming many-step approach, in this work a scalable single-step laser-writing methodology is demonstrated to produce ZnO/laser-induced graphene (LIG) composites by the irradiating a zinc-covered polyimide polymer with a CO₂ infrared laser under ambient conditions. This method offers a prompt synthesis process, the ability to perform a patterned design by using a computer-assisted laser-scribing system, the production of flexible samples and an easy scalability in a time- and cost-effective way. Additionally, since the formation of both components takes place simultaneously, an intricate close link develops between the two materials.

The resulting composites evidenced the presence of spherical-shaped ZnO crystalline microparticles randomly dispersed throughout the surface of LIG, whose amount, distribution and dimensions depended on the used laser processing conditions. Raman spectroscopy exposed the existence of both wurtzite-ZnO and sp² carbon, while the presence of the typical ZnO luminescence signal was assessed by photoluminescence. These measurements revealed a broad emission band in the visible range, as well as the characteristic near band edge emission in the UV spectral region. Furthermore, the energy peak position of the broad visible band was seen to slightly increase with laser power, highlighting the influence of the processing conditions on the composites’ defect distribution.

Presented @ Materiais 2019

Presenter: Joana Rodrigues

Authors: J. Rodrigues, D. Smazna, N. Ben Sedrine, E. Nogales, B. Mendez, R. Adelung, Y. K. Mishra, M. R. Correia, T. Monteiro

Although widely studied, zinc oxide (ZnO) micro and nanostructures, and particularly their composites with other materials, such for instance carbon nanostructures, remains a hot topic in the nanoresearch community. Owing to the possibility to combine the properties of the different components, new functional materials with more advantageous properties than each component by itself can be synthesised. Thus, the fundamental study of such properties is crucial to the understanding of the materials’ potentialities. For instance, size and morphology may play an important role in the composite’s properties. Moreover, ZnO surfaces are known to exhibit a large number of unsaturated bonds resulting in a high density of surface states. For structures with a high surface to volume ratio, the influence of the surface becomes relevant and these states will influence the material’s optical properties. Surface passivation may suppress the trapping of carriers by such states, affecting the semiconductor’s optical response.

In the present work, ZnO tetrapods (TPs) and microwires (MWs), both produced by flame transport method, were coated with C₆₀ molecules. The formed composites were characterised by Raman spectroscopy, photoluminescence (PL), cathodoluminescence and PL excitation. The optical response of the resulting ZnO/C₆₀ composites was compared with the ZnO reference samples both for TPs and MWs, enabling to conclude that the presence of C₆₀ changes the light outcome in both structures. It is also interesting to note that some differences in the luminescence signal arise from the two types of composites. For instance, in the case of TPs, the UV emission could only be observed at low temperature, presenting a low relative intensity when compared to the visible broad band, even when covered with the C₆₀ molecules. On the other hand, for the MWs the UV emission intensity is comparable with the visible one at room temperature but dominates the spectra at 14 K. The predominance of the visible band in the TPs structures may constitute an indication of the involvement of surface-related defects in its origin, since the surface contribution is expected to be higher in this case. In both cases, redshift of the peak position of the visible band was observed by increasing the C₆₀ content, being more pronounced in the case of the MWs. These results suggest the possibility to control/tune the visible emission outcome by increasing the C₆₀ amount on the ZnO surface. An adequate understanding of such phenomena may have quite beneficial implications when sensing applications are envisaged.

Presented @ Materiais 2019

• Nicole Yaacoub, “Conversão de ZnO em fosfato de zinco e sua viabilidade como transdutor em biossensores óticos” (“Conversion of ZnO to zinc phosphate and its viability as optical transducer in biosensors”) - Master Thesis in Physical Engineering, University of Aveiro 2021-2022, ongoing;

• Carolina Rodrigues, "Biossensores óticos para deteção de analitos biomédicos", ("Optical biosensors for the detection of biomedical analytes") - Master Thesis: Integrated Master in Physical Engineering, 2020-2021; Univ. Aveiro - ongoing;

• Mariana Brás, "Nanoestruturas de ZnO para monitorização de poluentes ambientais", ("ZnO nanostructures for monitoring environmental pollutants") - Master Thesis: Integrated Master in Physical Engineering, 2019-2020; Univ. Aveiro;

• Julia Zanoni, "Desenvolvimento e caracterização de compósitos ZnO/grafeno induzido por laser para aplicação em biossensores", (“Development and characterisation of ZnO/laser induced graphene composites for biosensing applications”) - Master Thesis: Integrated Master in Physical Engineering, 2018-2019; Univ. Aveiro.

• Daniela Santos, “Biosensores electroquímicos para deteção de glucose”, Degree in Biomedical Engineering, 2021-2022, Univ. Aveiro, ongoing.

• Ana Sofia Assunção, “Estudo de grafeno induzido por laser como transdutor eletroquímico para deteção de glucose”, 3^rd year research project: Integrated Master in Physical Engineering, 2019-2020, Univ. Aveiro - defended at 24/09/2020;

• Gabriel Marques, "Biossensores óticos baseados em nanoestruturas de ZnO para deteção de glucose"- 3^rd year research project: Integrated Master in Physical Engineering, 2019-2020, Univ. Aveiro, defended at 27/07/2020;

• Carolina Rodrigues, “Nanoestruturas de ZnO e nanocompósitos de ZnO/carbono para aplicação em biossensores óticos”- 3^rd year research project: Integrated Master in Physical Engineering, 2018-2019, Univ. Aveiro, defended at 24/07/2019;

• Ana Luísa Bento, “Preparação e funcionalização de nanocompósitos ZnO/grafeno induzido por laser para biossensores eletroquímicos de colesterol”- 3^rd year research project: Integrated Master in Biomedical Engineering, 2018-2019, Univ. Aveiro, defended at 12/07/2019;

• Maria Simone Soares, “ZnO eletrodepositado em grafeno induzido por laser para biossensores impedimétricos de PSA”- 3^rd year research project: Integrated Master in Biomedical Engineering, 2018-2019, Univ. Aveiro, defended at 12/07/2019.