p (Phys.org) - Sondas bioeletrônicas miniaturizadas transformam a biologia e a medicina, permitindo a medição de componentes intracelulares na Vivo . Recentemente, cientistas da Universidade de Harvard e da Universidade de Pequim projetaram, fabricou e demonstrou sondas bioeletrônicas tão pequenas quanto 5 nanômetros usando uma única heteroestrutura nanofio-nanotubo de três dimensões. (Uma heteroestrutura combina múltiplas heterojunções - interfaces entre duas camadas ou regiões de semicondutores cristalinos dissimilares - em um único dispositivo.) Por meio de medições experimentais e simulações numéricas, os pesquisadores mostraram que esses dispositivos têm resolução de tempo suficiente para registrar os sinais elétricos mais rápidos em neurônios e outras células, com integração em matrizes de chips maiores, potencialmente fornecendo mapeamento de ultra-alta resolução da atividade em redes neurais e outros sistemas biocelulares. p O prof. Xiaojie Duan discutiu o artigo que ela, Pesquisador Graduado Tian-Ming Fu, Prof. Charles M. Lieber e seus co-autores publicados em Proceedings of the National Academy of Sciences . Ela primeiro aponta que as sondas de nanotubos e sua heterojunção com transistores de efeito de campo de nanofios de silício (SiNW FETs) tornam-se mecanicamente menos estáveis ​​à medida que o diâmetro é reduzido. "Quando o nanotubo fica cada vez menor, "Duan disse ao Phys.org, "fica mais fácil quebrar o nanotubo na área de junção com o SiNW. Na aplicação do uso da sonda para detecção bioeletrônica intracelular, haverá várias forças, como a força capilar do líquido, bem como a interação entre a sonda e a membrana celular. Essas forças podem quebrar a sonda se houver uma junção fraca entre ela e o SiNW. "

p Outro problema é que a sensibilidade elétrica também é reduzida à medida que o diâmetro do nanotubo diminui, porque o diâmetro interno do nanotubo (ID) define a área efetiva da porta do dispositivo. "No registro do potencial transmembrana intracelular usando nossa sonda, "Duan explica, "citosol preenche o nanotubo e atua como o eletrodo de porta para o SiNW FET subjacente." Citosol (também denominado fluido intracelular ou matriz citoplasmática ) é o líquido encontrado dentro das células, excluindo organelas e outros componentes citoplasmáticos. "A alteração do potencial do citosol modula a densidade de portadores do SiNW FET, alterando assim sua condutância, "Duan continua." É assim que nossa sonda funciona para o registro bioeletrônico. "A área de contato entre o citosol e o SiNW - definida pelo diâmetro interno do nanotubo - determina a eficácia da modulação da condutância. Em outras palavras, se o diâmetro interno do nanotubo for muito pequeno, a área do portão SiNW FET também será muito pequena.

p Os pesquisadores também foram apresentados ao fato de que a resposta dinâmica de alta frequência pode degradar com a diminuição do diâmetro interno do nanotubo devido ao aumento da resistência da solução no nanotubo. "Quando o diâmetro interno do nanotubo diminui, "Duan explica, "a resistência da solução dentro do nanotubo aumentará, devido à diminuição da seção transversal do condutor da solução. "Além disso, a velocidade na qual o SiNW FET subjacente pode responder a um sinal é determinada pelo produto RC da capacitância e da resistência do condutor da solução dentro do nanotubo - então, se o nanotubo está ficando menor, a resistência está ficando maior. Isso significa que o SiNW FET precisará de mais tempo para responder - e se a mudança de sinal for muito rápida, a sonda não será capaz de gravá-lo de forma confiável. "Isso é o que queremos dizer com 'a resposta dinâmica de alta frequência se degradará com a diminuição do diâmetro interno do nanotubo, 'Duan acrescenta, "Contudo, descobrimos que para a nossa sonda, mesmo se diminuirmos o diâmetro interno do nanotubo para tão pequeno quanto 5 nm, a sonda ainda é capaz de registrar fielmente um sinal de 3 kHz - uma largura de banda suficiente na maioria dos casos para registrar sinais neurais e cardíacos. "

p Os pesquisadores também foram confrontados com a necessidade de usar detectores de transistores de efeito de campo de nanofios semicondutores ativos para superar as limitações da redução do tamanho da sonda. "A palavra ativo 'é comparada à natureza' passiva 'da gravação com eletrodos de metal, " Duan points out. "For metal electrode recording, a portion of the transmembrane potential V _m will be dropped or lost at the electrode/electrolyte interface, so the signal recorded will be smaller than the real transmembrane potential." When the size of the metal electrode is reduced, the impedance value at the electrode/electrolyte will increase – and at some point, this impedance will get so large that the recorded signal will be obscured by noise. Contudo, for the FET recording, the cytosol potential change is reflected by the semiconductor channel conductance change, which is independent of the probe/electrolyte interface impedance. "Since decrease in probe size will therefore não affect the signal amplitude, " Duan adds, "using the FET to sense potential is a very effective way to overcome the limitations of probe-size reduction."

p Another critical challenge was synthetically integrating nanotubes and nanowires. "The nanotube is made of silicon oxide and the nanowire is made of silicon, " Du an notes. "For cell recording, the nanotube needs to be built vertically onto the nanowires – meaning that a three-dimensional heterostructure is needed." While the heterostructure could be fabricated in various ways, not all of them provided the required controllability at the probe scale. Portanto, the researchers grew a germanium nanowire (GeNW) on top of the silicon nanowires, with the GeNW acting as a template for the nanotube. "Depositing SiO ₂ on the GeNW/SiNW heterostructure, then selectively removing the core GeNW, resulted in the desired nanotube/SiNW structure, " notes Duan. "Using this method, the nanotube inner diameter can be controlled by the GeNW diameter, the outer diameter can be controlled by the SiO ₂ thickness, and the nanotube length can be defined by the GeNW growth time. This gives us complete and easily implemented control over the probe's dimensions."

p Finalmente, the scientists had to investigate and model the bandwidth effect of phospholipid coatings, which are important for intracellular recording. "We use phospholipid coating to assist the nanotube probe to penetrate the cell membrane, " Duan notes. "Since the bandwidth of our probe is important for the probe to be able to record fast neural or cardiac signal, we need to make sure this phospholipid modification will not overly affect the bandwidth." Because the phospholipid layer will decrease the cross section of the solution conductor inside the nanotube, it impacts bandwidth in two ways by changing the capacitance and resistance of the solution inside the nanotube. (The phospholipid-modified probe bandwidth is easily determined by applying a fast artificial signal to the solution, and then recording how the conductance of the device changes over time. This provides the time the device needs to respond to this signal and thus the device's bandwidth.) "Therefore, " Duan explains, "if the nanotube is large, this thin phospholipid layer will not cause too much of a difference. Contudo, for our probe, the sub-10 nm nanotube size is almost the same scale as the lipid layer – so we have to carefully examine how it affects probe bandwidth, both experimentally and theoretically."

p The scientists addressed these myriad challenges in designing, fabricando, and demonstrating the probe with three key innovations. "The first is the use of FET as potential sensing element, which in principle enables us to overcome the size limit on the probe." Duan explains. "However, FETs have conventionally existed in a linear geometry with connections that preclude access to the inside of cells." The second innovation was the solution to this problem – namely, the design of a vertical SiO ₂ nanotube on top of the nanoscale FET, which allowed them to introduce cytosol into the cell without having to insert the FET channel inside the cell, which would be more invasive. The third key is the design of a relatively large nanotube base with a much sharper nanotube tip, resulting in a sub-10 nm probe without sacrificing its mechanical strength and electrical sensitivity.

p An important aspect of the study was ensuring that the bioelectronic devices had sufficient time resolution to record the fastest electrical signals in neurons and other cells. "The signals in neural system are normally in the millisecond scale, " Duan points out. "That means that to reliably record these signals, the recording device needs to have a bandwidth measure in kilohertz." While the probe's bandwidth decreases with the decrease of nanotube diameter, the scientists found that even for probes with inner diameters as small as 5 nm, the bandwidth is still around 3 kHz (a time resolution ~ 0.3 ms) in physiological solution. "This means that our probes have sufficient time resolution to record the fastest electrical signals in neurons and other cells, " Duan adds.

p Além disso, Duan points out, the scientists found that measuring the cell transmembrane resting potential with these ultrasmall bioelectronic devices demonstrates the capability for intracellular electrophysiology studies. "When we measured the transmembrane resting potential of HL-1 cell with our new probes, we found that with the phospholipid modification, the nanotube can easily and reliably penetrate the cell membrane, allowing the FET to record the intracellular transmembrane potential at full amplitude." After retracting the nanotube from the cell, the recorded potential can immediately revert to the extracellular potential. "Reliable cell membrane penetration and stable recording of intracellular transmembrane potential prove the capability of our probes for intracellular electrophysiology studies, " notes Duan.

p Seguindo em frente, says Duan, the researchers' are planning to scale up their work to integrate the probes into high-density, large-scale array for large-scale mapping of neural activities; use the probes to record neural signals from small subcellular structures/organelles; and investigate other applications in which the probes will provide substantially greater spatial resolution and minimal invasiveness than other techniques.

p Além disso, the scientists might consider developing other innovations. "Por exemplo, " Duan illustrates, "a major challenge in using our ultra-small probes for recording from small subcellular structures is to accurately position them with respect to the subcellular structures of interest. We're looking at either labeling our probe with fluorescence dye – or other biocompatible materials – to mark the nanotube at high resolution, or using specific targeting in which the probe's biochemical surface groups define the specific cell location being studied."

p Duan sees other areas of research that might benefit from their study, Incluindo:

• Neuroscience:Since recording of intracellular action potential and other low frequency transmembrane potential signal is important to study the neural network.
• Medicine:By studying how the drugs affect the recorded intracellular action potentials or other transmembrane potentials, the probes can provide an attractive technique for parallel or high-throughput screening of drugs targeting the ion channels.
• Biophysics:The process of cell membrane penetration by the phospholipid-modified nanotube can serve as a useful platform for studying the cell membrane's fusion-related biological processes.

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