Credit only if required

Featuring work from Prof. Yan Xu’s NanoBio Interfaces
Laboratory (NaBIL) at Osaka Prefecture University, which has
been involved in developing nanobio interfaces in nanofluidic
devices for single molecule and single cell studies.

As featured in:

Title: Site-specific nanopatterning of functional metallic andmolecular arbitrary features in nanofluidic channels
A general method for site-specific nanopatterning of functional
metallic and molecular arbitrary features in glass nanofluidic
channels was established. The method opens the way for precise
functionalization of nanofluidic channels, which has been greatly
challenging in the field of nanofluidics.
See Yan Xu et al., Lab Chip, 2015,
15, 1989.

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Site-specific nanopatterning of functional metallic
and molecular arbitrary features in nanofluidic
channels†
Yan Xu,*a Nobuhiro Matsumoto,ab Qian Wu,ac Yuji Shimataniac and Hiroaki Kawatab
We established a versatile method for site-specific nanopatterning of functional metallic and molecular

Received 16th February 2015,
Accepted 9th March 2015

arbitrary features in glass nanofluidic channels, with well-controlled feature sizes ranging from tens to hundreds of nanometers and precisely controlled placements in the range of several tens of nanometers. With
the method, we achieved the fabrication of quasi-0D, quasi-1D, 2D, and 3D gold nanopatterns in nano-

DOI: 10.1039/c5lc00190k

fluidic channels, as well as a high-density fluorescent molecular nanoarray in arrayed femtoliter nanofluidic
channels. The method opens the way for precise functionalization of nanofluidic channels, which has beenwww.rsc.org/loc

greatly challenging in the field of nanofluidics.

Nanofluidics, the study of fluids in channels with nanometerscale dimensions, is a potential solution to a variety of challenges in areas of separation, sensing, and detection of molecules at the single-molecule level.1–3 Nanofluidics is also
promising because it provides tools for elucidation of transport phenomena, chemical reactions, biological processes,
and life phenomena confined in nanoscale spaces.4–7 Nevertheless, in contrast to its thriving ‘brother’ microfluidics,
nanofluidics is still a nascent field. The lack of methods forthe functionalization of nanofluidic channels is one of themost critical bottlenecks for promoting both fundamental
and applied research on nanofluidics.
The functionalization of nanofluidic channels is challenging. For open planar systems, many physical, chemical, andmechanical methods have been developed to functionalize thesystems at zero, one, two or three dimensions in the nanometer
range.8–14 Unfortunately, for small closed space systems like
nanofluidic channels, most of these methods face obstacles
in either principles or practices due to the physical barrier
resulting from the ‘closed’ feature of these systems.
aNanoscience and Nanotechnology Research Center, Research Organization forthe 21st Century, Osaka Prefecture University, 1-2, Gakuen-cho, Naka-ku, Sakai,
Osaka 599-8570, Japan. E-mail: y-xu@21c.osakafu-u.ac.jpbDepartment of Physics and Electronics, Graduate School of Engineering, Osaka
Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japanc
Department of Chemistry, Graduate School of Science, Osaka Prefecture
University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan† Electronic supplementary information (ESI) available: Schematic drawing ofphoto-patterning widely used in microfluidics (Fig. S1†); nano-in-nano fabrication guided by the high-precision placement control technique (Fig. S2†); evaluation of precision of the placement control technique (Fig. S3†); long-term evaluation of the gold-pattern-friendly bonding under continuous nanofluidic
conditions at a high pressure (Fig. S4 in the ESI†); and Experimental section.
See DOI: 10.1039/c5lc00190k

This journal is © The Royal Society of Chemistry 2015

As the space size decreases to the micro-/nanometer scale,the surface-area-to-volume ratio increases significantly. As aresult, the properties of the internal surface of a small closed
space dominate a variety of phenomena and processes in thespace. Hence, fabrication, modification, and control of thelocal internal surface are indispensable strategies for thefunctionalization of small closed space systems. Surface patterning is a basic way to fabricate, modify, and control theproperties of the local surface of a material. In the field of
microfluidics, photo-patterning (Fig. S1 in the ESI†) is a predominant method to pattern the internal surface of a closed
microfluidic channel,15–18 in addition to few other patterning
methods.19,20 On the one hand, the method takes advantageof the nature of light (commonly, UV light), as light can passthrough the transparent physical barrier of the substrate layer
and thereby enables patterning of various photo-reactive
materials on the internal surface of the “closed” microfluidic
channel, using a photomask. On the other hand, the patterning capability of the method is restricted by the nature oflight, namely, the diffraction limit, as well as the resolution
limit of the photomask and other instrumental limitations.
Consequently, the feature sizes and the placement precisionof the pattern are generally in the micrometer range and practically difficult to be further downscaled to the nanometer
range. Obviously, nanopatterning of the internal surfaces ofnanofluidic channels exceeds the capability of photo-patterning. Therefore, in order to accelerate and broaden the
research on nanofluidics, methods capable of nanopatterningin nanofluidic channels should be established.
Unlike the patterning-after-bonding strategy of the photopatterning in microfluidics (Fig. S1, in the ESI†), in this study
a different general methodology (Fig. 1) was proposed toachieve nanopatterning in nanofluidic channels. The method

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Technical Innovation

comprises primary nano-in-nano patterning before bonding by
utilizing top-down approaches and secondary molecule patterning after bonding by utilizing bottom-up approaches. The
method does not use UV light, and therefore the capability ofpatterning is not restricted by the diffraction limit of lightand the resolution limit of the photomask. The method
enables the site-specific fabrication of a variety of nanopatterns with arbitrary shapes and sizes in single nanofluidic
channels, nanofluidic channel networks, and arrayed nanofluidic channels. Not only metallic but also molecular nanopatterns and nanoarrays can be formed in nanofluidic channels by utilizing the method.
In brief, the method (Fig. 1) is described as follows. First,
gold is pre-nanopatterned in open nanofluidic channels utilizing top-down processes of electron beam (EB) lithography,
deposition, and lift-off which are guided by a high-precision
placement control technique. Then, the gold-nanopatterned
open nanofluidic channels are bonded without damage utilizing a gold-pattern-friendly bonding technique to form closed
nanofluidic channels. Next, a solution of functional molecules bearing thiol or disulfide groups is introduced into thenanofluidic channels. Consequently, a functional molecular
nanopattern specifically forms on the surface of the gold
nanopatterns in the nanofluidic channels by bottom-up
molecular self-assembly, owing to well-known thiol–gold or
disulfide–gold interactions.21,22
The method includes two key techniques to surmount two
hurdles in the establishment of the novel patterning method,
described as follows.
One hurdle was how to precisely pattern gold in open
nanofluidic channels. As 3D hierarchical nanostructures
formed by dissimilar materials, open glass nanofluidic channels with gold nanopatterns cannot be fabricated by a onestep process utilizing current available nanofabrication
approaches. Therefore, a two-step process consisting of (i)nanofabrication of nanofluidic channels followed by (ii) gold
nano-deposition on exact locations in the open nanofluidic
channels in a controlled manner (i.e., nano-in-nano patterning; Fig. S2a and b in the ESI†) is unavoidable but is greatly
challenging.
This hurdle was surmounted by a placement control technique with super high precision. The technique was based on
accurate positioning of the fabrication placement assisted bya pair of gold reference marks pre-fabricated on the substrate. The gold reference marks could be searched manuallyand recognized by using a backscatter detector of an EB system when being scanned with the electron beam, according
to a backscattered electron yield different from that of theglass substrate. The rotational deviation and the positional

Fig. 1 Schematic drawing of nanopatterning in nanofluidic channels.

1990 | Lab Chip, 2015, 15, 1989–1993

Lab on a Chipdeviation during substrate mounting were thereby detected,
calculated with respect to the actual position, and offset bydriving an X–Y stage of the EB system. As a result, the relative
positional relationship between the reference marks on
computer-aided design (CAD) data and those on the actual
sample could be finally matched, which enabled superposition drawing with high precision during multiple-step nanofabrication using the EB system. Reference marks with different shapes and sizes were tested. Symmetrical cross-marks ofgold/chromium (Au/Cr, 100 nm/5 nm thick) with several tensof nanometers in width and several hundreds of nanometersin length were utilized in this study, based on the fabrication
capability and reproducibility of our fabrication facilities. By
using the reference marks, a placement control with superhigh precision of several tens of nanometers in both X-axis
and Y-axis directions was achieved, as indicated in Fig. S3 in
the ESI.† For example, the super high-precision placement
control enabled the site-specific fabrication of a very challenging nanopattern whose one side (Fig. 2c) or both upperand lower sides (Fig. 2d, h) is exactly located at the edges ofthe bottom of the nanofluidic channels (800 nm wide and
300 nm deep), without any protrusion of gold to the outsideof the nanofluidic channels. Moreover, the fabricated goldreference marks were quite robust even after experiencing EB
lithography (EBL), dry etching, and various harsh liquid processes of pattern development, metal lift-off, and substratecleaning several times. The gold reference marks could work
efficiently for nanofabrication with more than two steps,
practically. For example, Fig. 2j shows a scanning electron
microscope (SEM) image of a gold-bottomed nanowell arrayin arrayed nanofluidic channels. The structure is a nano-innano-in-nano structure, which was fabricated by three-step
nanofabrication (Fig. 2j, according to a process described inFig. S2c and d in the ESI†).
Another hurdle was how to bond gold-patterned nanofluidic channels without destroying the pre-patterned gold.
While there are several substrate materials such as glass,polydimethylsiloxane (PDMS), and plastics to choose in the
fabrication of microfluidic chips, glass (here referring specifically to fused-silica which is glass consisting of silica in theamorphous form) is currently almost the sole ideal substratematerial in the fabrication of nanofluidic chips. This is owing
to its chemical and mechanical stability, superior optical andthermal properties, and well-established nanofabrication
technologies. In standard practice, nanofluidic chips are fabricated by fusion bonding of upper and lower glass substrates
containing micro-/nanofluidic channels and other structures.23 However, the glass fusion bonding is required to be
performed under vacuum at over 1000 °C, which is near themelting point of gold under atmospheric pressure. The ultrahigh temperature would inevitably lead to damage of the gold
patterns.
Gold-pattern-friendly bonding of glass nanofluidic chips
was developed to surmount the second hurdle. We havedeveloped room temperature bonding of glass nanofluidic
channels based on one-step surface activation using O2/CF4

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Fig. 2 Schematic and SEM images of gold nanopatterns with arbitrary structures fabricated in (a–e) single nanofluidic channels, (f) nanofluidic
channel networks, and (g, h, and j) arrayed nanofluidic channels. (a) Nanodots; (b) nanowires; (c) nanostripes; (d) gradient nanopatterns; (e)diamond, square, triangle, and heart-shaped nanopatterns; (f) rectangular nanopatterns in cross-shaped nanofluidic channel networks with varying
X distances (50 nm, 100 nm, 200 nm, 300 nm, and 400 nm) which are defined in the corresponding schematic image; (g) a nanodot array inarrayed nanofluidic channels; (h) a full-width nanosegment array in arrayed nanofluidic channels; (i) a nanowell array (AFM image) and (j) a goldbottomed nanowell array in arrayed nanofluidic channels. White scale bar is 1 μm and black scale bar is 10 μm.

plasma treatment.24 The O2/CF4 plasma treatment is suitablefor bonding bare nanofluidic channels but is not friendly forbonding gold-nanopatterned nanofluidic channels. The problem is that etching and fluorination effects on the gold surface during the O2/CF4 plasma treatment could lead to damage of the gold, as reported elsewhere.25,26 To avoid thispotential problem, in this study a different surface activation
process using the O2 plasma without addition of CF4 was proposed. Briefly, first, the perfectly cleaned substrates weretreated with the O2 plasma for 15 min, two times, at a powerof 200 W and a flow rate of 100 mL min−1. After that, the substrates were preliminarily bonded by being in contact witheach other, with the use of hands, at room temperature,
followed by thermal bonding under vacuum. The process
allowed the gold-nanopatterned nanofluidic channels tobond under vacuum conditions of several 100 Pa, at 600 °C.
Most importantly, the gold patterns survived after bonding
and no damage was observed (Fig. 3d). The results should beowing to the fact that the bonding temperature was much
below the melting point of gold. Moreover, the bonded nanofluidic chip could work continuously without leakage duringliquid introduction driven by air pressure (Fig. 3e), even at

This journal is © The Royal Society of Chemistry 2015

430 kPa for six days (Fig. S4 in the ESI†). This pressure range
can meet the requirements of most nanofluidic operations.
Furthermore, as shown in Fig. 3f and g, the gold nanopatterns (Fig. 3d) in the bonded nanofluidic chips (Fig. 3a, b)
kept their ability to interact with disulfides to form SAM on
the nanometer scale. This result suggests that the bonding
did not bring about adverse influence which could lead to
failure in the formation of self-assembled monolayers (SAM)
on the gold surface.
The nanopatterning supported by the above-described two
key techniques enabled site-specific fabrication of gold nanopatterns in glass nanofluidic channels, with actual dimensions approximately the same as the targeted dimensions.
Fig. 2 shows the SEM or atomic force microscope (AFM)
images of nanodots (290 nm; Fig. 2a), nanowires (66 nm
wide; Fig. 2b), nanostripes (570 nm wide; Fig. 2c), and nanowells (700 nm square and 200 nm deep; Fig. 2i) with gold
bottoms (600 nm square; Fig. 2j) fabricated in nanofluidic
channels (800 nm wide and 300 nm deep), representing
quasi-0D, quasi-1D, 2D and 3D nanopatterns, respectively.
These reproducible nanostructures had feature sizes ranging
from tens to hundreds of nanometers. These nanostructures

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Fig. 3 (a) Formation of the fluorescent molecular nanoarray in gold-nanopatterned nanofluidic channels of (b) a chip by introducing a fluorescein
disulfide solution with (c) an air pressure-based liquid introduction system. (d) A bright-field image of the gold-nanopatterned nanofluidic channels
before the liquid introduction and (e) a fluorescence image of the channels after continuous liquid introduction for 2 h. (f) A bright-field image and(g) a fluorescence image of the formed fluorescent molecular nanoarray, with (h) a profile of fluorescence intensity of the first row of (g).

are typical structures widely used in a variety of systems innanoscience and nanotechnology, but were fabricated for the
first time in nanofluidic channels. Multiple patterns with
arbitrary shapes in a single nanofluidic channel were alsoachieved. For example, while Fig. 2d shows the SEM image ofa series of gold patterns with various gradient lengths(890 nm, 520 nm, 330 nm, and 160 nm) in a single nanofluidic channel (800 nm wide and 300 nm deep), Fig. 2e demonstrates the SEM image of diamond, square, triangle, andheart-shaped gold nanopatterns in a single nanofluidic channel (680 nm wide and 300 nm deep). Moreover, the ability ofsite specificity was further demonstrated by fabrication of ananometer-level-positioning-required structure in a crossshaped nanofluidic channel (520 nm wide and 240 nm deep),as shown in Fig. 2f. The rectangular gold nanopattern(300 nm × 470 nm) positioned at a specified distance (forexample, 50 nm, 100 nm, 200 nm, 300 nm, and 400 nm) withrespect to the boundary of a branch of the nanofluidic channel network was precisely fabricated as desired. In addition,these nanopatterns could be fabricated as nanoarrays inarrayed nanofluidic channels, as shown in Fig. 2g, h, and j.

1992 | Lab Chip, 2015, 15, 1989–1993

A self-assembled molecular nanoarray in arrayed nanofluidic channels was achieved in a nanofluidic chip by utilizing the nanopatterning method (Fig. 3a, b). The nanofluidic
chip contains a micro-/nanofluidic channel hybrid, which isa standard chip structure used in nanofluidics to facilitate
liquid handling in nanofluidic channels.24,27–29 In the chip,
two side microfluidic channels (3 μm deep and 500 μm wide)are bridged by 100 parallel-arrayed nanofluidic channels(300 nm deep, 800 nm wide, and 400 μm long, spaced by
2 μm), with a gold nanoarray (500 nm square, spaced by
2 μm) in each nanofluidic channel. The structure provides a
160 × 100 (= 16 000) nanodot-shaped gold array on an area of
400 μm × 280 μm (= 0.112 mm2) in the chip, as partiallyshown in Fig. 3d. Each nanofluidic channel has an ultrasmall volume of approximately 96 fL (femtoliter (fL) = 10−15
L). Thus, the arrayed nanofluidic channels are appropriately
to handle 9.6 pL (picoliter, pL = 10−12 L) of liquid, which isthe same as the volume of a single mammalian cell. All liquids were introduced to the arrayed nanofluidic channels
through the microfluidic channels by air pressure which was
regulated using a 0.1 kPa resolved pressure control system

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(Fig. 3c). An ethanol solution of HOOC–ĲCH2)10–S–S–ĲCH2)10–
CONH–fluorescein was introduced (Fig. 3e) to form the molecular nanoarray by molecular self-assembly in the nanofluidic
channels. HOOC–ĲCH2)10–S–S–ĲCH2)10–CONH–fluorescein is a
fluorescein disulfide which can form SAM on the gold surface
via disulfide–gold interactions. It is widely used to visualize
the defects and ordering of the SAM structure, owing to its
easily detectable fluorescence. A high-density nanoarray (160 ×100) of the fluorescein disulfide SAM was confirmed on theentire 400 μm × 280 μm tiny area of the arrayed nanofluidic
channels by using a fluorescence microscope. Fig. 3f and gshow a bright-field image and a corresponding fluorescence
image of a part of the molecular nanoarray, respectively. Fig. 3hshows the profile of fluorescence intensity of the first row of
the array shown in Fig. 3g. The nanodot-shaped fluorescentarray (Fig. 3g) with a high resolution (Fig. 3h) indicates thatthe disulfide molecules were successfully self-assembled on
the gold nanoarray in the nanofluidic channels. The fluorescence of the nanoarray remained detectable after continuous
flowing of absolute ethanol by air pressure at 430 kPa, for 18 h.
These results reveal that the molecular nanoarray was stableeven under high-pressure fluidic conditions for a long term.
In conclusion, we established a versatile method enablingsite-specific nanopatterning of metallic and molecular arbitrary features in glass nanofluidic channels, based on the proposed methodology comprising nano-in-nano patterning before
bonding and molecule patterning after bonding. To achieve
the method, we developed two key techniques, namely, theplacement control with a precision of several tens of nanometers for multiple-step nanofabrication and the gold-patternfriendly bonding of nanofluidic chips. With the nanopatterning supported by the two techniques, we achieved thefabrication of quasi-0D, quasi-1D, 2D, and 3D gold nanopatterns in nanofluidic channels as well as a high-density
molecular array in arrayed femtoliter nanofluidic channels
for the first time. These versatile nanopatterns are very promising because they can work as fluidic, electrical, optical, thermal, magnetic, chemical and biological functional units innanofluidic chips after further design, characterization, optimization, and integration in the future. In the field of nanofluidics, due to the lack of methods for functionalization of
nanofluidic channels, until now most studies have to stay at alevel of investigation of phenomena confined in less controllable, bare nanofluidic channels. Therefore, we believe that
nanopatterning will help accelerate and broaden both thefundamental and the applied research on nanofluidics.

Acknowledgements
This work was partially supported by JSPS KAKENHI (grantno. 26706010, 26630403), MEXT KAKENHI (grant no.
26107714), and the Asahi Glass Foundation.

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Technical Innovation

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