Generalized On-demand Production of Nanoparticle Monolayers on

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Generalized On-demand Production of Nanoparticle Monolayers on Arbitrary Solid Surfaces via Capillarity-mediated Inverse Transfer Jeehan Chang, Jaekyeong Lee, Andrei Georgescu, Dan Huh, and Taewook Kang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00248 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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Generalized On-demand Production of Nanoparticle

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Monolayers on Arbitrary Solid Surfaces via

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Capillarity-mediated Inverse Transfer

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Jeehan Chang1, Jaekyeong Lee1, Andrei Georgescu2, Dan Dongeun Huh2, and

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Taewook Kang1*

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1Department

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04107, Korea

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2Department

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of Pennsylvania, Philadelphia, PA 19104, USA

of Chemical and Biomolecular Engineering, Sogang University, Seoul,

of Bioengineering, School of Engineering and Applied Science, University

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*[email protected]

ACS Paragon Plus Environment

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ABSTRACT

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Century-old Langmuir monolayer deposition still represents the most

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convenient approach to the production of monolayers of colloidal nanoparticles on solid

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substrates for practical biological and chemical sensing applications. However, this

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approach simply yields arbitrarily-shaped large monolayers on a flat surface, and is

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strongly limited by substrate topography and interfacial energy. Here we describe a

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generalized and facile method of rapidly producing uniform monolayers of various

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colloidal nanoparticles on arbitrary solid substrates by using an ordinary capillary tube.

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Our method is based on an interesting finding of inversion phenomenon of a

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nanoparticles-laden air/water interface by flow through a capillary tube in a manner that

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prevents the particles from adhesion to the capillary sidewall, thereby presenting the

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nanoparticles face-first at the tube’s opposite end for direct and one-step deposition

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onto a substrate. We show that our method not only allows the placement of a

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nanoparticle monolayer at target locations of solid substrates regardless of their surface

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geometry and adhesion, but also enables the production of monolayers containing


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nanoparticles with different size, shape, surface charge, and composition. To explore

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the potential of our approach, we demonstrate the facile integration of gold nanoparticle

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monolayers into microfluidic devices for real-time monitoring of molecular Raman

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signals under dynamic flow conditions. Moreover, we successfully extend the use of our

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method to developing on-demand Raman sensors that can be built directly on the

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surface of consumer products for practical chemical sensing and fingerprinting.

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Specifically, we achieve both the pinpoint deposition of gold nanoparticle monolayer and

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sensitive molecular detections from the deposited region on clothing fabric for detection

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of illegal drug substance, a single grain of rice and an orange for pesticide monitoring,

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and a 100-dollar bill as a potential anti-counterfeit measure, respectively. We believe

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that our method will provide unique opportunities to expand the utility of colloidal

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nanoparticles and to greatly improve the accessibility of nanoparticle-based sensing

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technologies.

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KEYWORDS


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Colloidal nanoparticles, Capillarity, Two-dimensional monolayer, Inverse transfer,

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Optical detection, Lab on a chip

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The unique combinations of magnetic, electronic, and optical properties

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observed in large two- or three-dimensional assemblies of colloidal nanoparticles have

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garnered considerable research attention1-9. In particular, such assemblies at an

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interface have been accomplished against thermal diffusion of individual nanoparticle by

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utilizing covalent bonding between capping ligands10,11, solvent evaporation12,13,

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external stimuli (e.g., electric field, magnetic field, light, or flow)14-20, and interfacial self-

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assembly21-23. Among these methods, interfacial self-assembly, which is driven by the

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reduction of interfacial energy through the entrapment of nanoparticles at fluid

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interfaces24-26,

can

easily

afford

large-area

and

two-dimensional

nanoparticle

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monolayers27,28. Since these monolayers at the fluid interfaces are highly susceptible to

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destabilization and the resultant loss of structural integrity due to external perturbations,

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the transfer of such monolayers onto certain solid substrates is more desirable for

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practical use such as sensing applications. However, the paucity of robust means for

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production and practical implementation of uniform nanoparticle layers on various solid

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substrates remains challenging issue.


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To date, the most widely used technique to this end is Langmuir-

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Blodgett/Langmuir-Schaefer film deposition that utilizes simple immersion of solid

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substrates into nanoparticle-containing liquids in an either parallel or perpendicular

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orientation to the interface after the interfacial assembly of colloidal nanoparticles29-34.

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This approach does not require precise control of colloidal evaporation, nor does it rely

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on lithographic patterning of solid substrates as required in evaporation-based methods.

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Despite its advantages, however, Langmuir deposition has several critical limitations

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that hamper its widespread use for practical sensing applications. For example, it is

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limited for the fabrication of nanoparticle monolayers either on part of solid surfaces to

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be probed or on solids with complex surface geometry (e.g., fruit, fabric, customer

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products with a non-flat surface). In addition, this approach does not allow one to

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reproducibly generate multiple spots of nanoparticle monolayers on the same surface,

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which may be required for multiplexed sensing applications. Although microcontact

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printing has been proposed as an alternative35,36, it is often limited for producing uniform

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nanoparticle monolayers on biological surfaces which can be damaged by high

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pressure applications. Furthermore, elastomeric stamps used in this method are prone


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to deformation, swelling, and shrinkage during the stamping of nanoparticle assemblies

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onto solid substrates. A considerable loss of nanoparticles due to preferential adhesion

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to the stamp itself rather than the substrate is another inherent limitation of this

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technique that has yet to be overcome.

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To suggest a new approach to address these major technical challenges, here,

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we describe a robust and facile method of rapidly producing uniform monolayers of

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various colloidal nanoparticles on arbitrary solid substrates by using an ordinary glass

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tube. Our proposed method consists of the formation of two-dimensional assembly of

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colloidal nanoparticle at an air/water interface and, more importantly, following inverse

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transfer via reconstitution of the monolayer interface along the direction of gravity by a

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capillary tube. This interfacial reconstitution which permits one-step transfer of the

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nanoparticle monolayer from the air/water interface to any solid surface without applying

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a pressure is enabled by exploiting capillary rise and inversion phenomenon in the tube.

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Our approach provides compelling advantages over previous methods including

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Langmuir deposition by offering (i) the compatibility with a wide range of solid substrates


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with varying surface topography and physicochemical properties (our method is also

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compatible with biological surfaces), (ii) the flexibility of depositing various types of

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nanoparticles on a solid substrate at a precise location, and (iii) the convenience of

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manual operation with high degrees of repeatability and consistency. Moreover, this

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method permits rapid and cost-effective production of nanoparticle monolayers on solid

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substrates without requiring specialized facilities and personnel.

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Our technique to transform colloidal nanoparticles to their uniform monolayers on

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arbitrary substrates is outlined in Figure 1 and Supporting Video 1. Initially, a small

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amount of ethanol is added to an aqueous colloidal solution of nanoparticles to form a

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two-dimensional nanoparticle assembly at an air/water interface. When a glass capillary

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tube is gently immersed tip-first into the air/water interface supporting the nanoparticle

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monolayer, water flows upwards by capillary action into the tube while maintaining the

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nanoparticle monolayer at the rising interface. Removal from solution followed by

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inversion of the capillary tube causes the contained water plug to be drawn by gravity to

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the opposite end of the tube, thereby exposing the interfacial monolayer at the open


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face of the capillary. By subsequently bringing the monolayer-presenting face of the

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capillary into contact with a solid substrate in a downward spotting motion, the

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monolayer can be immediately transferred in its entirety by virtue of van der Waals

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interactions irrespective of whether the surface is soft, smooth, rough, curved, fibrous,

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porous, adhesive, or even exposed to the direction of gravity. Importantly, the

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straightforwardness of this technique enables robust reproducibility without being

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overshadowed by an intrinsic dependence on extraordinary manual dexterity or a highly

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practiced operator. During repetition, it is unaffected by minor technique variations to

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immersion depth and time, inversion speed, or spotting duration, which is useful to

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maintaining the technique’s reliability and repeatability between the hands of multiple

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users or the clutches of multiple automated machines.

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Figure 2 shows representative results of nanoparticle monolayers produced by

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our capillary transfer technique. The sampling of a monolayer region from a large two-

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dimensional assembly of gold nanoparticles at the air/water interface using a glass

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capillary tube is demonstrated in Figure 2a. Upon vertical contact of the capillary tube


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with the nanoparticle-laden interface, we observe the upward, capillary driven flow of

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water that maintains the sampled nanoparticle monolayer at the top of the rising column,

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the presence of which is indicated by the slightly darkened yet specular surface on the

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concave meniscus (Figure 2b, the left-most). When the tube is subsequently inverted,

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the water plug spontaneously moves downwards due to gravity over the course of 2-4

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seconds to reach the opposite end of the tube, thereby exposing the monolayer at the

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open capillary face (Figure 2b, from left to right). Although the monolayer-supporting

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meniscus is still concave as it flows through the tube due to its strong hydrophilic

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interaction with the glass wall (Figure 2b, the fourth), the meniscus becomes planar or

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slightly convex when the interface becomes exposed at the opposite end of the tube

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(Figure 2b, the right-most); this facilitates nanoparticle transfer to the target solid

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surface by achieving direct contact of the monolayer-bearing interface without any

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additional application of force or pressure. The monolayer is immediately immobilized

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on the substrate upon direct contact due to van der Waals interactions, which are

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sufficiently strong to maintain the monolayer structure even after washing the surface

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with water (Figure 2c). Supporting Information Figure 1 shows a representative image of


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a 17-nm gold nanoparticle monolayer on a polydimethysiloxane (PDMS) sheet, acquired

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by scanning electron microscopy (SEM) after washing with water.

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Notably, the monolayer inside the capillary is maintained during transfer without

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any nanoparticle losses by adhesion to the glass wall, despite the Langmuir-

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Blodgett/Langmuir-Schaefer deposition processes’ tendency to exploit the adhesion

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between nanoparticles and glass. This structural stability of the monolayer can be

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attributed to the action of a lateral capillary force between the monolayer and the glass

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wall mediated by the water phase. In ethanol-induced assembly of colloidal metal

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nanoparticles at liquid interfaces, ethanol is generally known to not only reduce

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electrostatic repulsion between the particles but also make the surface of the

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nanoparticle surfaces more hydrophobic37. As shown in Figure 2d, the contact between

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water and both the hydrophobic nanoparticles and the hydrophilic glass wall would

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produce lateral capillary force between the monolayer and the glass wall that prevents

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any nanoparticle-to-glass adhesion. Despite small Bond number of the nanoparticle, the


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resulting lateral immersion capillary force is orders of magnitude higher than thermal

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fluctuation38,39.

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Characterization of the transferred monolayers was performed by measuring

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their optical properties using surface-enhanced Raman mapping. To perform these

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measurements, a monolayer of 50-nm gold nanoparticles was deposited on a PDMS

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substrate using a capillary with an inner diameter of 1 mm (Figure 3a, b). As shown in

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SEM (Figure 3c) and atomic force microscope (AFM) (Supporting Information Figure 2)

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images, a uniform monolayer of densely-packed gold nanoparticles is observed. The

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substrate was then immersed into 10 mM ethanolic 2-naphthalenethiol (2-NAT) solution

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for 10 min, which was followed by Raman mapping measurements. Figure 3d presents

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a Raman intensity mapping image obtained at Raman transition of 1,058 cm-1 (assigned

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to the symmetric C-H bending vibration of 2-NAT40), which revealed uniform Raman

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intensity over the full extent of the deposited monolayer without significant variations in

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signal intensity; this result is indicative of a high degree of structural uniformity

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(Supporting Information Figure 3). In addition to the Raman transition at 1,058 cm-1, we


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further analyzed Raman intensity at 1,379 cm-1 which is assigned to the ring stretching

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vibration of 2-NAT40 from various locations (i.e., 315 locations) in the same monolayer

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region (Figure 3e). As shown in Figure 3f, the measured relative standard deviations for

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both Raman transitions are determined to be only 4.8 and 4.7%, respectively. To

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investigate process variability, nine deposited monolayers of 50-nm gold nanoparticles

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were evaluated at the 1,058 cm-1 transition. As shown in Figure 3g, h, the nine

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monolayers exhibit uniform structures (Supporting Information Figure 4) and Raman

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intensity maps, and in addition, the monolayers produce near-identical UV-vis spectra

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(Figure 3i). Taken together, our method is capable of producing structurally uniform

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monolayers with constant far- and near-field optical properties (Supporting Information

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Figure 5), which implies a high degree of consistency in analytical measurements made

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from the deposited monolayer.

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To validate the results of our technique relative to traditional methods of

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nanoparticle deposition, we deposited gold nanorod on glass slides by employing (1)

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covalent bonding between amine group-modified glass and the gold nanorods, (2)


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colloidal evaporation, and (3) our method, and compared their surface-enhanced

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Raman intensities for rhodamine 6G (Supporting Information Figure 6). As shown in

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Figure 3j, the enhancement factor for the gold nanorod monolayer that is produced by

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our method is estimated to be 2.5 × 104, which outperforms the other methods by 1-2

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orders of magnitude. This difference is mainly ascribed to variations in the density and

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morphology of gold nanorods in a probed area. As shown by SEM in Figure 3k, covalent

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bonding-mediated deposition results in irregular gold nanorod attachment with low

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particle density, while colloidal evaporation produces non-uniform aggregates rather

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than a uniform monolayer.

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Next, we investigated our ability to tune the diameter of transferred

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nanoparticle monolayers by using differently sized glass capillaries in the procedure.

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Note that the shape of monolayers transferred is not limited to circular-shape

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(Supporting Information Figure 7). As expected and shown in Figure 4a, the size of the

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uniform monolayer of 50-nm gold nanoparticle is linearly proportional to the diameter of

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the capillary. Notably, our method allows the selective production of uniform monolayer


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regions as large as 1.4 mm and as small as 100 μm in diameter with minimal

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irregularities (including voids or cracks). Although nonuniformities within deposited

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monolayers were observed using capillary tubes of diameters larger than 1.4 mm, we

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expect the method to be compatible with highly uniform patterning of spots smaller than

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the 100 μm minimum size that we tested, provided a sufficiently thin capillary is sourced.

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In addition to size, the layout of the monolayers deposited by our method is

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also highly amenable to tuning. For example, our method is capable of concurrently

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depositing multiple nanoparticle monolayers if capillaries are bundled together, either in

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direct contact or with spacers; for example, we demonstrate the fabrication of either

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trigonal, tetragonal and hexagonal arrays of monolayers in one step by using multiple

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tubes at the same time (Figure 4b). A complex array of monolayers (i.e., the letters

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“NRG,” Figure 4c) may also be deposited for applications including multiplexed sensing,

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either by using one tube sequentially or an array of tubes in a custom holder.

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To examine the capability of our method to deposit nanoparticle monolayer

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onto solid surfaces of various textures and compositions, monolayers of 50-nm gold


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nanoparticles were transferred onto a collection of solid substrates by using a 1-mm

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capillary tube. A polished silicon wafer and a glass slide were chosen because their

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surfaces are hydrophobic and hydrophilic, respectively, whereas a fibrous paper and a

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porous polycarbonate membrane were selected as a result of their coarse surface

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roughness and complex geometry. Photographs in Figure 4d show that monolayers of

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uniform size are produced on all substrates tested, with corresponding SEM images in

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further indicating that the gold nanoparticles are closely packed in each of the

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monolayer regions and no irregular aggregates or void spaces are observed. These

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results suggest that our method is well suited for the fabrication of sensors for paper- or

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porous membrane-based optical detection41-43.

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Our method is also compatible with biological surfaces that can be damaged

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by high pressure application due to the fluidic and flexible nature of liquid interfaces. To

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examine this compatibility, as represented in Figure 4e, we delivered gold nanoparticles

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on the surface of bacteria (i.e., Escherichia coli). The substrate was simply prepared by

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evaporation of highly concentrated E. coli solution on a PDMS substrate. As shown in


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Figure 4f (right, SEM image taken for the inside of the monolayer region), gold

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nanoparticles monolayer is transferred onto E. coli by monolayer thickness while

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maintaining E. coli’s cylindrical cellular structure (Figure 4f, middle and left). However,

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surface deformation of E. coli and significant loss of gold nanoparticles are observed

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when nanoparticles monolayer is transferred by using PDMS stamp (Supporting

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Information Figure 8).

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To investigate whether our method is widely applicable to various types of

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colloidal nanoparticles, nanoparticles with different sizes, shapes, compositions, and

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surface charges were tested. More specifically, 17- and 50- nm gold nanoparticles, gold

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nanorods, 30- and 45-nm silver nanoparticles, and gold-silica core-shell nanoparticles

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were used. Citrate-capped gold nanoparticle and hexadecyltrimethylammonium

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bromide-capped gold nanorod are well known to exhibit negative and positive surface

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charges, respectively. These nanoparticles were also characterized by UV-vis

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spectrophotometry (Supporting Information Figure 9). Figure 4g shows colloidal

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assemblies of various nanoparticles at an air/water interface before being transferred,


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and the resulting monolayers on glass slides as insets. It is important to note that the

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nonuniform appearance of the colloidal assemblies at the air/water interface before

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being transferred is due to the fact that the kinetics of their assembly at the air/water

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interface generally depends on the size, shape, concentration, and surface wettability of

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nanoparticles. SEM images in Figure 4g clearly show uniform and densely-packed

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monolayers of the above nanoparticles without noticeable aggregates.

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The capability of our method to produce dense monolayers of nanoparticles on

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a solid substrate with complex geometry would be very useful in microfluidic optical

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detection technique that utilizes nanoparticles for signal enhancement and sensor

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transduction in lab-on-a-chip platforms. As a proof-of-concept, we deposited

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monolayers of 50-nm gold nanoparticles in several microfluidic designs such as typical

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of dual-inlet, Y-shaped lab-on-a-chip biosensors, some with upstream mixing elements,

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and some with expanded detection zones located near the downstream outlet to

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accommodate larger nanoparticle monolayers. These channels were fabricated from

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PDMS elastomer casting, then spotted on their channel interior with a nanoparticle


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monolayer using our deposition method, and finally bonded to a glass slide to create an

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enclosed channel (as depicted in Figure 5a). The resulting channels are presented in

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Figure 5b and Supporting Information Figure 10, with the integrated with gold

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nanoparticle monolayers visible as darkened spots; for the leftmost pair of microfluidic

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channels that possess larger detection-zone widths, monolayers were transferred using

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a 1-mm capillary, and a smaller 0.6-mm capillary tube was used for the remaining

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rightmost pair of channels.

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To be suitable for lab-on-a-chip applications testing large fluid volumes at high

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flow rates through miniaturized microchannels, the surface-deposited monolayers must

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withstand the resultant elevated shear stresses under flow conditions and remain

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immobilized without shedding from the channel surface. The stability of the deposited

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particles was examined by increasing the flow rate from 500 to 2,000 μl/min (Figure 5c).

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As shown in SEM results in Figure 5d, no significant detachment of gold nanoparticles

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was observed after 30 min, irrespective of the flow rate used. To further confirm this

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structural stability, Raman intensities of rhodamine 6G bands at 1,363 and 1,509 cm-1


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assigned to aromatic C-C stretching (Supporting Information Figure 11) were recorded

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from the monolayers under constant flow conditions; uniform Raman intensities for both

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transitions were observed at all flow rates (Figure 5e). This structural stability under

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dynamic flow conditions can be attributed to the fact that our method generates dense

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monolayers of nanoparticles upon deposition, rather than clumped aggregates with a

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tendency to detach or roll under a similar condition, increases per-particle adhesion to

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the microchannel surface and permitting flow rates sufficiently high.

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Our method is also directly scalable to optical sensing applications that require

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fast, nondestructive, flexible, and portable analyte characterization, which range from

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drug and explosive inspection, food safety monitoring, to anti-counterfeiting. As a proof-

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of-concept, for a model drug inspection application, we successfully detected trace

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levels of benzocaine (chosen as a surrogate of cocaine) on clothes by exploiting the

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capability of our method to produce uniform nanoparticle monolayers onto fibrous

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substrates. Specifically, trace benzocaine solution was dropped on a fabric and allowed

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to dry, which was followed by the transfer of 50-nm gold nanoparticle monolayer (Figure


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6a, left). Compared to the Raman spectrum of solid benzocaine, characteristic peaks of

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benzocaine at 850, 1,159, and 1,594 cm-1 are selectively observed from the monolayer

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region in the fabric (Figure 6a, right).

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Next, we also achieved the delivery of monolayers onto a single grain of rice

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(Figure 6b) and an orange (Figure 6c). Prior to transfer, the above substrates were

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treated with trace commercial pesticide solution (10-5 M chlorpyrifos-methyl solution)

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and dried. Then, monolayers of 50-nm gold nanoparticles were transferred onto the

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substrates by using a 1-mm diameter capillary. As shown in Raman spectra of Figure

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6b, characteristic Raman transitions of chlorpyrifos-methyl at 677, 1,004, and 1,438 cmwhich are assigned to C-Cl stretch, P-O-R stretch, and ring vibrations44, respectively,

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1

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are selectively observed for the monolayer-coated substrate regions on a grain of rice.

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Similarly, characteristic Raman transitions of chlorpyrifos-methyl at 1,141, 1,291, and

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1,436 cm-1 (C-H deformation, ring mode, ring vibrations44) are also exclusively observed

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for the monolayer-coated substrate regions on orange peel (Figure 6c). Limits of

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detections (LODs) are found to be 10-5 and 10-6 M (Supporting Information Figure 12),


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respectively which are much lower than suggested concentration level for practical use

2

(0.7 - 1.4 mM). Thus, these results suggest that our method can be used to fabricate

3

sensors for rapid and sensitive monitoring of trace hazardous chemicals in foods.

4

Finally, 50-nm gold nanoparticle monolayers were deposited on multiple surface

5

features of an American 100-dollar bill. The monolayers are intentionally deposited onto

6

several different locations on the bill, since the monolayers with a unique Raman

7

transition can serve as a chemical identifier to prevent counterfeiting. As shown in

8

Figure 6d and Supporting Information Figure 13, upon illumination with a 785-nm laser,

9

a strong peak around 250 cm-1 (assigned to Au-O stretching of Au-COO- moieties)

10

appears exclusively from the monolayer-coated regions.

11

We have described a versatile and widely applicable one-step method to

12

fabricate uniform and large monolayers of various functional nanoparticles on arbitrary

13

solid substrates that vary in both texture and material properties using off-the-shelf

14

glass capillary tubes. The method is robust and repeatable in the hands of different

15

users owing both to its high reproducibility within a comfortable margin of technique


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variability, as well as the intrinsic simplicity of inverting a capillary tube and spotting onto

2

a target position. The physical basis of this method lies in the inversion of a

3

nanoparticle-laden air/water interface by flow through a capillary tube in a manner that

4

protects the particles from adhesion to the capillary sidewalls, thereby presenting the

5

nanoparticles face-first at the tube’s opposite end for direct deposition onto a surface

6

with full control of position. The size of the transferred monolayer can be controlled in

7

the range of hundreds of micrometers to over one millimeter by simply changing the

8

capillary tube diameter among the large assortment available commercially. Analysis of

9

resultant deposited monolayers by SEM images, Raman mapping, and UV-vis

10

measurements indicate that our method allows the deposition of dense and highly

11

uniform nanoparticle monolayers on diverse types of solid surfaces with variable

12

roughness, softness, and geometry. Additionally, we demonstrate the method’s

13

ubiquitous applicability to depositing uniform monolayers of nanoparticles possessing

14

different sizes, shapes, surface charges, and compositions. We have also demonstrated

15

several potential applications of our method, including the facile integration of gold

16

nanoparticle monolayers into microfluidic devices for continuous Raman monitoring


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under rapid fluid flow that exert high shear. Moreover, monolayers of gold nanoparticles

2

are successfully deposited on a 100-dollar bill, a single grain of rice, an orange, and a

3

piece of fabric. Raman signals of the monolayer and surrounding molecules such as

4

benzocaine are shown to be detectable exclusively for monolayer-coated regions. We

5

anticipate our method to significantly extend the practical utility of colloidal nanoparticles

6

to a multitude of sensing applications that require flexible, simple-to-use, portable, and

7

highly reproducible characterization techniques as well as a consistent, repeatable

8

fabrication strategy.


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ASSOCIATED CONTENT

2

Supporting Information.

3

The Supporting Information is available free of charge on the ACS Publications website.

4

Experimental section; A representative SEM image; Atomic force microscope

5

(AFM) measurements; A Raman map of gold nanoparticle monolayer and

6

following Raman intensity; Crack area measurements; Average Raman

7

intensities over entire nine monolayers; Surface-enhanced Raman spectroscopy

8

(SERS) spectra of R6G using 3 different deposition methods; Transfer using

9

rectangular-shaped capillary tubes; Comparision with microcontact printing; UV-

10

vis spectra of various colloidal nanoparticles; Photographs of various microfluidic

11

channels integrated with nanoparticle monolayers; Detection of R6G in

12

microfluidic channel via SERS; Detection of commercial pesticide solution

13

(chlorpyrifos-methyl) on a single grain of rice and orange peel; Raman spectra

14

measured on various locations in 100-US-dollar bill; Schematic video of

15

capillarity-mediated inverse transfer onto solid surfaces.


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2

AUTHOR INFORMATION

3

Corresponding Author

4

*E-mail: [email protected]

5

Author Contributions

6

T.K. conceived the concept. J.C., J.L., and T.K. designed and organized the

7

experiments. J.C. and J.L. performed the experiments. J.C., A.G., and D.H. contributed

8

to the fabrication of microfluidic channels. All authors discussed the results and wrote

9

the manuscript. The authors are also thankful to Professor Luke P. Lee for helpful

10

discussion.

11

Funding Sources

12

This research was supported by the Mid-Career Researcher Support Program (No.

13

2016R1A2B3014157) and C1 Gas Refinery Program (No. 2018M3D3A1A01055759)

14

through the National Research Foundation of Korea funded by the Ministry of Science,

15

ICT, and Future Planning.


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Notes

2

The authors declare no competing financial interest.

3

4

REFERENCES

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478.

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Kim, D.; Kwon, H. J.; Shin, K.; Kim, J.; Yoo, R. E.; Choi, S. H.; Soh, M.; Kang, T.; Han. S. I.; Hyeon, T. ACS Nano 2017, 1, 8448-8455.

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M.; Yang, H.; Murray, C. B.; Kagan, C. R. Nano Lett. 2017, 17, 1387-1394.

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Ortiz, M. N.; Rodal-Cedeira, S.; Costas, C.; Celiksoy, S.; Pérez-Juste, I.;

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Scarabelli, L.; La Porta, A.; Pérez-Juste, J.; Pastoriza-Santos, I.; Scarabelli, L.

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2

Figure 1. (a) Schematic representation for the generalized and facile method for

3

fabricating uniform monolayers of various colloidal nanoparticles on arbitrary solid

4

surfaces including biological surfaces by using an ordinary capillary tube. (b) Rapid and

5

selective delivery of colloidal nanoparticles on various customer products (clothing

6

fabric, rice, and orange) for chemical screening and fingerprinting. Scale bar is 3 mm.


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Page 34 of 43

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Figure 2. Selective and rapid transfer of a nanoparticle monolayer on a solid via

3

capillary rise and inversion of an ordinary glass tube. (a) Photographs of two-

4

dimensional gold nanoparticle monolayer formed from colloidal nanoparticles (inset) and


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selective separation of the monolayer at an air/water interface by using a glass capillary

2

tube. (b) Photographs of the water plug and nanoparticle monolayer inside the capillary

3

tube during inversion of the tube. Blue arrows indicate the position of nanoparticle

4

monolayer in capillary and letters ‘W’ and ‘A’ correspond to the water plug and air,

5

respectively. (c) Transfer of the monolayer-presenting end of the capillary onto a PDMS

6

substrate by vertical contact. (d) Illustration for repulsive force between glass wall and

7

nanoparticle monolayer resulting from the surface deformations.


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Page 36 of 43

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Figure 3. Uniformity and reproducibility of transferred gold nanoparticle monolayer. (a) A

3

photograph of transferred 50-nm gold nanoparticle monolayer on a PDMS substrate. (b)


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An enlarged photograph of transferred monolayer. (c) A representative SEM image of

2

transferred monolayer. (d) A Raman mapping image of transferred monolayer using the

3

intensity of the 1,058 cm-1 after immersion in 10 mM 2-NAT solution for 10 min. (e) All

4

Raman spectra within the monolayer. (f) Raman intensities at 1,058 and 1,379 cm-1 in

5

Raman mapping images. (g) Enlarged photographs of 9 nanoparticle monolayers

6

transferred from the same batch. (h) Raman mapping images using the intensity of the

7

1,058 cm-1 of 9 gold nanoparticle monolayers. (i) UV-vis spectra of 9 monolayers. (j)

8

Log(EF)s according to the three different methods for applying plasmonic nanoparticles

9

to optical detection system based on SERS: (1) Self-assembled monolayer (SAM)-

10

based deposition of gold nanorods (GNRs), (2) evaporation-mediated deposition of

11

GNRs, and (3) capillary force-mediated deposition of GNR monolayer. (k) Photographs

12

(left row) and SEM images (middle and right row) of the substrates for (1), (2), and (3).

13

Scale bar (left row) is 0.5 mm.


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Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

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Figure 4. Versatile production of monolayers of various nanoparticles on generic solid

3

substrates with tunable monolayer size. (a) A bright-field microscope image (leftmost)

4

and photographs of transferred 50-nm gold nanoparticle monolayer on glass substrates

5

with increasing tube diameters. (b) Manual production of multiple identical nanoparticle

6

monolayers at one time by using an assembled bundle of capillary tubes. Scale bar is 3

7

mm. (c) A ‘NRG’ pattern on a PDMS substrate of nanoparticle monolayers which are


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transferred in a series. Scale bar is 5 mm. (d) Photographs (top row) and SEM images

2

(bottom row) of transferred 50-nm gold nanoparticle monolayer onto various solid

3

substrates. Scale bars are 1 mm (top inset) and 100 nm (bottom inset). (e) Schematic

4

illustration for our inverse transfer of 50-nm gold nanoparticle monolayer onto E. coli. (f)

5

Representative SEM images for the outside (left), the borderline (middle), and the inside

6

(right) of the monolayer region, respectively. Scale bars (inset) are 1 mm. (g)

7

Photographs of the monolayer of various nanoparticles on PDMS substrates (left inset)

8

transferred from their large two-dimensional assemblies at air/water interface (left) and

9

each SEM images (right). Scale bars are 2 cm (left), 0.5 mm (left inset), 500 nm (right),

10

and 100 nm (right inset), respectively.


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Page 40 of 43

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Figure 5. Facile integration of gold nanoparticle monolayers into microfluidic channels.

3

(a) Schematic illustrations for fabrication of microfluidic channels. (b) Photographs of

4

50-nm gold nanoparticle monolayer-deposited microfluidic channels with various

5

geometries. (c) Schematic illustration for plasmonic detection of rhodamine 6G in a Y-

6

shaped microfluidic channel (left and middle) and a representative SEM image of the

7

monolayer within the channel (right). Scale bar (right, inset) is 1 mm. (d) Photographs


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(inset) and SEM images of the monolayer after 30 min of 1 mM rhodamine 6G solution

2

flow with different rate from 500 to 2,000 μl/min. Scale bars are 500 nm and 1 mm

3

(inset), respectively. (e) Raman intensities at the characteristic transitions of rhodamine

4

6G (i.e., 1,363 and 1,509 cm-1) with respect to time.

5

6

Figure 6. Sensing applications to drug inspection, food safety monitoring, and anti-

7

counterfeiting. (a) Photographs of the monolayer on a fibrous substrate (left) and

8

following SERS-based detection of benzocaine (right). (b, c) Photographs of the


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Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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monolayer on a grain of rice (b, left) and orange peel (c, left) and following SERS-based

2

detection of commercial pesticide solution (10-5 M of chlorpyrifos-methyl) on each

3

surfaces (b, c, right). Black lines indicate background SERS spectra of monolayers on

4

pesticide-untreated surfaces. (d) Raman spectra obtained from different locations on a

5

100-US-dollar bill where 50-nm gold nanoparticle monolayers are deposited and the

6

same nanoparticle monolayer on other paper and glass substrates (rightmost).


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TOC graphic

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Generalized On-demand Production of Nanoparticle Monolayers on

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