Increased Refractive Index Sensitivity by Circular Dichroism Sensing

Subscriber access provided by ECU Libraries

C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Increased Refractive Index Sensitivity by Circular Dichroism Sensing through Reduced Substrate Effect Gunnar Klös, Matteo Miola, and Duncan Stewart Sutherland J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12152 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 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

The Journal of Physical Chemistry

Increased Refractive Index Sensitivity by Circular Dichroism Sensing through Reduced Substrate Effect Gunnar Klös1, Matteo Miola2 and Duncan S. Sutherland1* 1Interdisciplinary

Nanoscience Center (iNANO), Aarhus University,

Aarhus C 8000, Denmark

2Carbon

Dioxide Activation Center

(CADIAC) - iNANO, Aarhus University, Aarhus C 8000, Denmark

ABSTRACT

An optical sensor based on the localized surface plasmon resonance (LSPR) of chiral Au nano-hooks with increased refractive index (RI)

sensitivity

via

circular

dichroism

(CD)

measurements

is

presented. Programmed control of sample rotation combined with angled physical vapor deposition is applied to hole-mask colloidal lithography to provide in process modification of the hole-masks and generate arrays of chiral nanostructures with an adjustable optical response. Extinction spectra with unpolarized light as well as circular dichroism measurements are compared for left and

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 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 2 of 39

right handed hook structures. Analysis of the LSPR peak shift of the substrate attached nanostructures revealed the CD measurements to be twice as sensitive as the measurements with unpolarized light (304 nm RIU-1 and 146 nm RIU-1, respectively) and close to the maximum predicted for LSPR sensing at this spectral region (~700 nm).

Finite-difference

time-domain

(FDTD)

simulations

with

different substrate materials show that the difference in RI sensitivity

can be

attributed to

the

limiting

effect

of the

substrate for the unpolarized extinction measurements, while CD based

sensing

retains

a

high

sensitivity,

unaffected

by

the

limiting effect of the substrate. CD based readout could provide a complementary and improved sensitivity for substrate bound LSPR sensor formats.

MAIN TEXT

1. Introduction Biosensing is a rapidly growing field with an increasing demand for low-cost and real-time solutions in various areas like

drug-discovery,

evaluation,

point-of-care

proteomics

or

diagnosis

environmental

and

clinical

monitoring.1–3

A

special focus lies on optical biosensors for their capability to perform fast, quantitative and label-free analysis, while


2

Page 3 of 39 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


offering

the

possibility

of

multiplexing

or

miniaturization.1,4,5 The

field

of

nanophotonics

employs

nanostructured

metamaterials that can manipulate and amplify electromagnetic (EM)

fields

sensitivity case

of

at of

subwavelength

spectroscopic

subwavelength

scales

to

increase

measurements.6–9

oscillations

of

The

the

specific

electrons

in

such

structures, called plasmons, forms the basis of a growing number of optical biosensor applications.10 In particular, the highly sensitive spectral position of electronic localized surface

plasmon

refractive

index

resonances (RI)

is

(LSPRs) utilized

to for

the

nearfield

ultrasensitive

(picogram level) detection.11–13 . Circular dichroism (CD) is the differential absorption of left- and right-hand circular polarized (LCP and RCP) light by chiral matter.14 It is an important measurement technique in various fields, such as chemistry, physics and especially medicine

and

biology,

where

its

main

application

is

the

structural and conformational analysis of proteins and other macromolecules.14,15 As an optical spectroscopy method, it has the advantage of being faster and more scalable than other approaches like nuclear magnetic resonance (NMR) spectroscopy or crystallography but, due to the weak interaction between light and the active chiral molecular centers, it comes with


3


the

inherent

disadvantage

of

low

Page 4 of 39

sensitivity,

typically

measuring CD signals of only a few millidegrees (mdeg). Compared

to

molecular

promote

much

stronger

between

the

exciting promising

size

of

wavelength. candidates

chiroptical

systems, CD

the

plasmonic

signals, optically

Thus, for

measurements.

due

active

plasmonic

the

to

the

a

better

center

can match

and

the

nanostructures

are

sensitivity

However,

particles

15,16

enhancement

properties

of

of the

evanescent fields of plasmonic structures are intrinsically different

to

the

far

field

of

standard

spectroscopic

measurements, especially when working with circular polarized light (CPL).17,18 To be able to combine CD spectroscopy with plasmonic sensing the fundamentals of the interaction of plasmonic structures and CPL need to be understood to which end the field of chiral plasmonics has emerged.17,19,20 Recently, a lot of progress has been made in studying the fundamentals of chiral plasmonic structures and how they interact with CPL in the far- and nearfield and also how they interact with chiral and achiral objects in their vicinity.6,17,18,21–25 These efforts already led to first applications in biosensing, polarization tuning and chiral catalysis.6,26 A special focus has been the emergence of superchiral fields in the nearfield of plasmonic structures, which potentially


4

Page 5 of 39 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


allow

the

direct

scaling

of

CD

measurements

down

to

the

nanoscale.6,27,28 However, since the interaction between CPL and plasmonic particles is strongly influenced even by minute structural

differences,

most

of

the

studied

plasmonic

structures are fabricated using time consuming and costly top-down methods such as electron-beam lithography.18,22,29–31 Nonetheless, various methods for the bottom-up fabrication of chiral structures have been demonstrated, employing e.g. the assembly of achiral plasmonic particles on chiral scaffolds or guided chiral assembly by the means of DNA origami.32–34 In particular the bottom-up method of hole-mask colloidal lithography (HMCL) has proven to be a reliable method for the large

scale

and

low-cost

production

of

plasmonic

biosensors.35–37 Even though it is an intrinsically achiral method

the

deposition

combination allows

for

with

the

e.g.

programmable,

fabrication

of

complex

angled chiral

structures.21–23,38 A very simple and straightforward chiral structure achievable with this method are 3D nano-hooks which were

shown

to

exhibit

strong

CD

responses

and

which

interactions with CPL have been studied in some detail.21,23,39 The

basic

plasmonic

modes

of

this

system

have

been

theoretically explained using electro-magnetic simulations by Frank et al. and Fang et al., with the latter also providing experimental

cathodoluminscence

data

to

experimentally


5


support their findings.

21,23

Page 6 of 39

Here we build up on this work by

further exploring the tunability of the plasmonic response offered by the fabrication method, specifically contrasting the excitation with CPL and unpolarized light. Furthermore, we provide a detailed study of the nano-hooks’ applicability as a refractive index sensor. A strong effect when using substrate-supported nano-particles (NPs)

for

reduction

sensing of

the

applications sensitivity,

is

the

caused

by

substrate portions

induced of

the

enhanced E-field being buried in the substrate.40–44 Various methods

to

lower

effect

of

the

substrate

have

been

demonstrate, mostly focusing on minimizing the contact area between NP and substrate.41,45 Alternatively, the NP geometry can be adapted to move the high field regions out of the substrate.46,47 For chiral nano-structure, the effect of the substrate can be crucial for the emergence of a CD signal, due to the induced symmetry breaking, but in how far the substrate influences the CD sensing capabilities has so far, to our knowledge, not been studied.48–51 Here, we are presenting a systematic study of a chiral Au nano hook and its potential for use in refractive index (RI) based biosensing

applications

and

show

measurements can give up to a

that

read-out

through

CD

two times enhancement in RI

sensitivity compared to measurements with unpolarized light. FDTD


6

Page 7 of 39 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


simulations show that the higher CD RI sensitivity is a result of an absence of a substrate effect, which significantly reduces the RI sensitivity for unpolarized light measurement. With the CD sensitivity

reaching

close

to

the

theoretical

limit

for

unsupported plasmonic resonators.47,52 2. Experimental Section Materials: Sample substrates were either 0.2 mm thick glass slides

(Menzel-Gläser)

for

spectroscopic

measurements

or

polished Si wafers for SEM and AFM measurements. The HMCL process involved PMMA (MW = 495,000, 4% in anisole, purchased from

Micro

resist

technology

GmbH),

poly(diallyldimethylammonium chloride) (PDDA, MW = 200,000350,000,

20%

in

styrenesulfonate)

H2O,

(PSS,

Sigma-Aldrich),

MW

=

70,000,

30%

poly(sodium-4in

H2O,

Sigma-

Aldrich) and Polyammonium chloride (PAX-XL60m Kemira Miljø). Colloidal

polystyrene

purchased

from

sulfate

Invitrogen

latex

(120

nm,

(PS)

particles

were

5.1%

coefficient

of

variation, 8.1% solids w/v). The bulk RI change measurements were performed with glycerol (99.5% pure, Sigma-Aldrich). Sample

fabrication:

Before

fabrication

substrates

were

cleaned either by 30 min ultrasonication in acetone (for Si wafers) or by wiping with acetone followed by 15 min in O2plasma (Vision 300 MK II, Advanced Vacuum) at RF power of 100


7


Page 8 of 39

W, 300 mbar pressure and 55 cm3/s oxygen flow (for glass substrates). An approximately 160 nm thick PMMA layer was deposited by a 1 min spincoating procedure at 3000 rpm with an acceleration of 1000 rpm/s followed by a 2 min baking step at

180°C.

Then,

a

5

min

UV

ozone

treatment

(UV/Ozone

ProCleanerTM Plus from Bioforce Nanosciences) was followed by the successive formation of self-assembled monolayers (SAMs) of PDDA, PSS and PAX, in that order, from diluted solutions (PDDA 2 wt% in MilliQ water (MQ), PSS 2 wt% in MQ, PAX 5 wt% in MQ). The layers were formed by keeping the samples for 30 s in solution and rinsing it afterwards with MQ for 1 min and drying the sample in a flow of nitrogen gas. The SAMs helped stabilizing the following formation of a monolayer of PS particles, deposited from 0.2 wt% solution for 2 min finalized by 90 s rinsing with MQ, drying in a nitrogen stream and 90 s UV ozone treatment. The hole-mask was formed by PVD deposition of 20 nm Ti (Ebeam thermal PVD, prototype, Polyteknik) with subsequent PS particle removal via tape stripping and O2 plasma etching for 22 min (RF 50 W, 25 mbar pressure, 40 cm3/s oxygen flow). Directly after the etching step the samples were transferred into

the

PVD

machine.

The

here

presented

chiral

hook

structures were achieved by PVD (2 Å/s) of Au at a GLAD angle of

16°

while

simultaneously

rotating

the

sample

holder.


8

Page 9 of 39 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


Rotation

speed

started

at

0.07

°/s

and

was

constantly

accelerated until it reached 0.25 °/s at which speed it was kept for the last 50° for a total rotation of 270° and a total deposition of 208.9 nm Au. After the Au deposition the sacrificial PMMA layer was removed in

a

>2

min

acetone

ultrasonication

step

and

ultrasonic

cleaning in ethanol/IPA and MQ for 30 s. Au nano-dots around the hook structure were removed by 12 min O2 plasma cleaning (RF 100 W, 30 mbar pressure, 55 sccm oxygen flow) and the Au was reduced for >8 h in HEPES buffer (10 mM with 100 mM NaOH, pH 7.4, Sigma-Aldrich). CD and unpolarized light UV-Vis spectroscopy: Spectroscopic analysis

with

CPL

was

performed

with

a

commercial

CD

spectrometer (Jasco-810 CD spectrophotometer, range 200 nm to 900 nm). Spectra with unpolarized light were recorded with a custom build fiber spectrometer (CypherTM PDA spectrometer from B&WTek, range 350 nm to 1050 nm) with a 20 W constant current tungsten light source (BPS2.0, B&WTek). For the bulk RI measurements the samples were mounted in a custom

build

flow

cell

with

a

double

sided

glass

wall.

Consecutively the sample were flushed with mixtures of MQ and glycerol with increasing RI, inserted via syringe. For the thin layer measurements, the samples were mounted in a rotational holder and for each sample and each layer at


9


least

four

spectra

with

varying

Page 10 of 39

rotation

were

taken

at

different spots on the sample. Each layer was added by PVD of 1.5 nm Al (0.5 Å/s) followed by a 30 s oxidation procedure in O2 plasma (RF 35 W, 60 mbar, 70 cm3/s oxygen flow). All thin layer measurements were performed in air. Simulations: commercial

All

FDTD

simulations solver

were

carried

Lumerical

out

solutions.

using

For

Au

the the

dielectric function from Johnson & Christie was used, for alumina the data was taken from Boidin et al., for the glass substrate and the surrounding medium constant RI values were set (Nglass = 1.52).66,67 In each case a mesh size of 2x2x2 nm3 was used and two perfectly matched layers at each side of the simulated

cube.

Perfectly

matched

layer

(PML)

boundary

conditions were used to account for the absence of long-range ordering

resulting

from

the

sparse

colloidal

lithography

fabrication approach. To simulate the response to unpolarized light,

the

separately

measured

spectra

for

two

perpendicularly oriented excitations with linearly polarized light were averaged. CD spectra were obtained in a similar fashion by calculating 𝐶𝐷 = atan

(

)

𝑅𝐻 ― 𝐿𝐻 𝑅𝐻 + 𝐿𝐻

for spectra obtained by

RH and LH CPL excitation. Charge distribution plots were created accordingly by averaging the charge distribution for


10

Page 11 of 39 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


the

two

perpendicular

polarized

excitations

and

by

subtracting the two CPL excitations. SEM,

AFM

and

XPS

measurements:

The

scanning

electron

microscopy (SEM) images presented here were obtained with the FEI Magellan 400 system, at an acceleration of 5 kV and a probe

current

of

50

pA.

Atomic

force

microscope

(AFM)

measurements were performed with the Bruker Dimension Edge with a RTESP-300 tip (front angle (15±2)°, back angle (25±2)°, side angle (17.5±2)°) in ambient conditions in tapping mode. X-ray photon spectroscopy (XPS) data was collected with a Kratos Axis UltraDLD. The X-ray source is monochromated Al kα (hν = 1486.6 eV) and operates at 15 mA (150 W) and 10 kV. The pressure in the chamber was kept below 5 × 10−9 mbar during measurements. The high resolution spectra for Au 4f were obtained with pass energy of 20 eV. The deconvolution of the spectra was carried out using the software CasaXPS. The gold peaks have been deconvoluted with asymmetrical 95% Lorentzian [A(0.4,0.1,0)Gl(95)] peak shape for Au (0) and symmetrical 95% Lorentzian peak shape for Au (III). 3. Results and Discussion 3.1. Fabrication of chiral nano-hooks via HMCL


11


Figure

1:

(a)

Schematic

Page 12 of 39

representation

of

the

nano-hook

fabrication by hole-mask colloidal lithography and programmable angled physical vapor deposition. (b) Typical SEM image of the achieved nano-hook array. The inset shows a high-resolution image of a single hook structure. (c), (d) 2D and 3D AFM image of nanohook structures. All scale bars are 100 nm.

In Figure 1 (a) the fabrication process for 3D Au nano-hook structures is schematically illustrated. The process is based on

the

standard

method

of

HMCL,

in

brief

a

sacrificial

Poly(methyl methacrylate) (PMMA) layer is spun coated onto a substrate, here either a glass slide for optical measurements


12

Page 13 of 39 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


or a silicon wafer for scanning electron microscopy (SEM) analysis.53

A

monolayer

of

colloidal

nanospheres

is

then

deposited from solution via electrostatic self-assembly onto a

previously

final

deposited

hole-mask

is

polyelectrolyte

then

created

triple

after

layer.

drying

by

The the

perpendicular physical vapor deposition (PVD) of a thin Ti layer and subsequent removal of the colloidal particles and then O2-plasma etching through the sacrificial layer. The result is a suspended sacrificial hole array mask that is ordered on a short range scale but without definite long range order, see Figure 1 (b). The surface density and size of the holes

are

suspension

determined and

of

by

the

the

properties

substrate.

For

of

the

colloidal

details,

see

the

experimental section. As previously mentioned, chiral structures can be achieved by combining the achiral hole-mask with angled PVD. By tilting the sample during PVD and simultaneously rotating it, the planar geometry of the resulting structures can be controlled. During deposition a clogging effect of the hole changes the deposition

footprint

and

can

be

controlled

through

the

deposition rate and rate of rotation to define precise 3D geometries of the resulting structure.54,55 With our equipment we

can

perform

a

unique

mode

of

angled

PVD

with

full

programmable control of the angle of deposition in plane and


13


Page 14 of 39

out of plane. We use this programmability to modify the speed of rotation during the deposition giving additional control over the clogging mechanism. We utilize the clogging effect and full control angled PVD to create a 3D hook structure that

decreases

in

height

and

width

along

its

circular

backbone, thus rendering it chiral (Figure 1, (c), (d)). For a precise control of the process, we previously determined the

materials

specific

clogging

rates

and

role

of

tilt

displacement (see Figure S1). To finalize the sample fabrication the sacrificial PMMA layer is removed by ultrasonic cleaning in acetone, followed by ethanol (or 2-Propanol) and MilliQ-water. SEM images of the samples at this point in the fabrication process revealed the main

structures

to

be

surrounded

by

much

smaller

Au

structures, known as the blurring effect of stencil masks (see Figure S2).56 To avoid nearfield interactions between the main

structure

and

these

satellite

structures,

they

are

removed by relatively strong O2-plasma treatment (100 W, 0.25 Torr,

12

min,

see

Figure

S2).

This

in

turn

requires

an

extensive reduction of the oxidized Au by storing it in buffer solution with high ionic strength for several hours. The final sample is shown in Figure 1 (b)-(d). As pointed out before, the versatile fabrication process allows for the control of various geometric parameters of the final


14

Page 15 of 39 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


structure, such as its height and width and how these parameters decrease along its backbone, as well as the radius of the circular backbone. To determine the optimal parameters for utilizing the nano-hooks as a sensor, we present in the following section a numerical analysis of the relation between the hook’s optical response and its structural parameters. 3.2. Structure plasmon relationship

Figure 2: Simulated CD of Au nano hook structure as represented in Figure S4. For all parameter sweeps, the basic parameters are a maximum height of H = 60 nm, a minimum height of 10 nm, a maximum width of w = 95 nm, a minimum width of 30 nm, backbone radius of R = 47.5 nm and a circular sectioning of θ = 240°. (a) Variation


15


Page 16 of 39

of the maximum width, w, from 75 nm to 95 nm in 5 nm steps. (b) Variation of the maximum height, H, from 10 to 50 nm in steps of 10 nm and of the minimum height from 1 nm to 8 nm. (c) Variation of the backbone radius, R, from 42.5 nm in steps of 5 nm to 62.5 nm. (d) Variation of the circular sectioning from 246° to 270° in 4° steps.

To understand the relation between the hook’s plasmonic modes and its structure we model it using FDTD simulations. The initial model parameters are determined from SEM and AFM analysis of the experimental structure, as shown in Figure 1. The main parameters of the models are the width and height of the hook and how these parameters decrease along its backbone, as well as the radius and circular sectioning of the backbone. Figure 2 shows how varying these main parameters influences the CD response of the hook (the influence on the unpolarized response is shown in Figure S3). We find that the impact made by the variation of the width of the hook is to influence the overall intensity of the response, where a wider structures yields higher intensity (Figure 2a). This can be readily explained by an increase in the overall amount of material and thus an increase in the number of electrons participating in the plasmon oscillations. Here it should be noted that the


16

Page 17 of 39 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


broad peak is an overlap of different plasmonic modes as can be seen in the emergence of minor peaks superposed on the broad response. Variation of the circular sectioning of the backbone show to slightly red-shift the plasmonic response when being increased (Figure 2d). This shift seems to be caused by changes in the interplay of individual plasmonic modes rather than an overall red shift of all modes. The strongest effect is observed for the variation of the hook’s height (Figure 2b). As for the increase of the width, also the increase in height amplifies the signal due to the added material/electrons. In addition to that, a strong blue-shift appears when making the structure taller. However, although this shift effects all plasmonic modes, its magnitude varies between the modes, separating them for less tall structures and

completely

merging

them

for

the

tallest

observed

structure. Similarly, for the variation of the radius of the circular backbone, a blue-shift appears when reducing the radius (Figure 2c). This shift seems to affect all modes in the same way. These results now allow determining the structural parameters necessary

to

enable

using

the

hook

in

a

biosensing

application. The main limitation is given by the experimental setup for collection CD spectra. Commercially available CD spectrometers,

as

present

in

most

laboratories

where

the


17


Page 18 of 39

structure of biomolecules is studied, have a spectral range from about 200 nm to 900 nm, where it should be noted that the range from 800 nm to 900 nm is affected by high levels of noise. Thus, for the structure being able to work as a sensor in

a

standard

laboratory,

it

needs

to

have

a

strong

CD

response in the spectral region below 800 nm. For the design of the hook structure, it follows that the optimal structure should be relatively tall with a small backbone radius. Both requirements

have

a

limitation

in

the

experimental

realization of the structure in form of a shadowing effect in the rotational center of the structure leading to piled up material in this location (see Figure 1). This observation was subsequently included in the simulations by adding an additional tall cone at the specific location, though it was found not to have a strong effect on optical response (see Figure S4).


18

Page 19 of 39 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


Figure 3: Comparison between the CD and unpolarized spectra of the two possible handedness of the hook structure (as shown in Figure 1 and Figure S7) for experiments (a) and simulations (b). (c) FDTD analysis of the plasmonic modes present in the visible spectrum of the LSPR response to CD and unpolarized light. Furthermore, the left hand (LCP) and right hand circular polarized (RCP) extinction spectra are shown, which constitute the CD spectrum, as well as the extinction spectra for two extinction spectra for parallel (II)

and

perpendicular

(ꓕ)

linearly

polarized

light,

which

constitute the unpolarized spectrum. The insets show the extracted charge distributions (each box: left = unpolarized, right = CD), for which more details are given in Figure S12.


19


In

Figure

3

the

spectroscopic

Page 20 of 39

analysis

of

the

finalized

experimental structure is compared with those from the FDTD model (left, top and bottom, respectively). In each of the cases both possible handedness of the hook structure are compared for their response to CPL as well as unpolarized light. The simulations show equivalency for both handedness in

each

measurement

experimental

mode,

results

which

within

the

is

mirrored

experimental

by

the

margins

of

error. It is noteworthy that the experimental spectra are located in a slightly lower wavelength region and appear to be

less

broad

than

the

simulated

spectra.

A

possible

explanation could be that we did not model the structure height

along

simulations,

the due

backbone

to

a

low

completely

spatial

correct

resolution

of

in

the

the

AFM

measurements performed on the experimental structure (see Figure 1 c,d). Additionally, the simulations are used to determine the different plasmonic modes contributing to both the CD and the unpolarized response. To that end, Figure 3 (c) displays how both spectra arise from the difference of the transmission spectra of LCP and RCP and the average of two perpendicularly oriented linear polarization directions. For the visible wavelength region, the simulations show a reasonable spectra,

with

agreement allowing

to

the

determine

experimentally the

measured

plasmonic

modes


20

Page 21 of 39 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


contributing

to

wavelengths,

the

the

broad

plasmonic

simulations

do

peaks. not

For

longer

replicate

the

experimental results exactly. This can be attributed to the high sensitivity of the plasmonic modes in this wavelength region to even minute structural variations as shown in Figure 2,

also

explaining

the

variance

in

signal

shape

between

experimental structures. In total three main modes are found, at 602 nm, 662 nm and at 740 nm. The one at the lowest wavelength

can

be

classified

as

a

dipole

mode.

It

only

contributes to the response to unpolarized light, thus causing the overall spectrum to be significantly broader than the CD response

(this

can

be

explained

by

the

Kauzman

model

of

chirality).57,58 The other two modes are of quadrupolar nature and contribute equally to both spectra, with the unpolarized response being slightly shifted towards the higher wavelength mode.

3.3. Bulk RI sensitivity and substrate effect


21


Page 22 of 39

Figure 4: Shift of plasmonic response caused by a change in bulk refractive index (RI) of the medium surrounding the hook. Exemplary spectra recorded for one hook sample for unpolarized light (a) and CD (c). Simulated unpolarized (d) and CD (f) spectra. LSPR peak shifts compared to the peak position at RI = 1.336 for experiments (b) and simulations with (e). Red lines are the shifts of the CD response, blue lines those of the unpolarized response, dashed lines indicated opposite handedness of the hook (identical for simulations). Comparing the effect of the substrate on the RI


22

Page 23 of 39 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


sensitivity with substrates (constant RI 1.54, data reproduced from e) and without substrate for the unpolarized (g) and CD (h) response, the corresponding spectra are shown in Figure S9. The maxima of 12th order polynomial fits to the top 20% of the peaks are used for determining the exact peak positions.

To quantify the sensing capability of the present chiral Au hook

structure

we

performed

measurements

of

the

spectral

response to changes in the bulk refractive index, both under CP and unpolarized illumination. Therefore, the samples were mounted in a flow cell setup and flushed with mixtures of water and glycerol with increasing glycerol concentrations. Hence, we were able to obtain spectra for varying the RI between 1.36 and 1.48. Figure 4 illustrates the obtained CD and unpolarized spectra exemplarily for one hook (a,c) as well as for the corresponding simulations (d,f). Furthermore, it shows the comparison between the LSPR peak shifts obtained from

the

simulations

and

the

experimental

spectra.

The

displayed errorbars are calculated from measurements on at least four different samples for each handedness of the hook. The experimental data shows that the peak shifts deduced from the CD spectra are significantly stronger than those from the unpolarized spectra, 304 nm RIU-1 compared to 146 nm RIU-1, with the former almost reaching the theoretical limit for


23


gold

plasmon

Simulations

resonances show

a

at

similar

this

Page 24 of 39

spectral

picture

at

position.47,52

slightly

lower

refractive index shifts. By comparing the spectra of the two handedness of the hook, we furthermore find that there is no measurable difference in neither the unpolarized nor the CD response.

A

possible

explanation

for

the

difference

in

sensitivity lie in the fact that CPL and unpolarized light excite different plasmonic modes, as shown in Figure 3c. The main

mode

of

the

CD

excitation,

although

at

a

lower

wavelength, has a more pronounced charge concentration in the sharp end of the hook, compared to the main mode of the unpolarized excitation. Using the simulations, we can compare the two respective contributions to the CD (LCP and RCP) and the

unpolarized

(parallel

and

perpendicular

linearly

polarized) response (Figure S10), we are able to show, that it is not one specific form of polarization that leads to the sensitivity increase. In fact, both, the response to LCP and RCP, are significantly less sensitive than the CD response. Interestingly, the response to the parallel (to the gap of the hook) linear polarized light has a higher sensitivity than the unpolarized response, which can be connected to its main plasmonic being located at the same spectral position as the CD mode.


24

Page 25 of 39 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


Since the RI sensitivity of the CD signal in this region is seen to be almost as large as that predicted for a plasmonic nanoparticle

far

away

from

a

substrate

while

the

linear

polarized signal had a RI sensitivity reduced by the presence of the substrate we explored the effect of the substrate RI on the RI sensitivity. The bottom row of Figure 4 compares the RI sensitivity of CD and unpolarized measurements for a substrate supported hook with a freestanding one, calculated by FDTD. Removing the substrate increases the sensitivity of the

unpolarized

measurements

significantly,

as

has

been

previously reported, while having only a minor influence on the sensitivity of the CD RI measurements.40,41,43 In the limit of the hook being immersed in a homogeneous RI environment, the unpolarized response is slightly more sensitive than the CD response. FDTD field distributions show that both CP and linearly polarised light have significant portions of the enhanced near field within the substrate precluding that the relevant nearfield of the CD signal is lifted out of the substrate. The CD signal results from the comparison of two signals (left and right circularly polarized light) and we interpret

the

lack

of

importance

of

the

substrate

in

determining CD based RI sensitivity to this normalisation of the signal to the available field for the shift.


25


Page 26 of 39

3.4. Decay length analysis

Figure 5: Shift of plasmonic response caused by the addition of thin layers of high refractive index (RI) material (Al2O3, RI ≈ 1.7). Layers were added in steps of 3 nm to a maximum thickness of 12

nm.

Exemplary

spectra

recorded

for

one

hook

sample

for

unpolarized light (a) and CD (c). Simulated unpolarized (d) and CD (f) spectra. Peak shifts compared to the peak position before addition of layer for the experiments (b) and for a 1 nm thick layer

for

the

simulations

(e);

this

distinction

is

further

explained in the main text. Red lines are the shifts of the CD response, blue lines those of the unpolarized response, dashed lines indicated opposite handedness of the hook (identical for


26

Page 27 of 39 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


simulations). The maxima of 12th order polynomial fits to the top 20% of the peaks are used for determining the exact peak positions. We analyzed the decay length of the field from the metal surface by extracting the E-field of the simulated structures for the plasmon mode at 740 nm. Figure S8 and Figure S11 depict the E-field of planar (S8) and vertical (S11) crosssections at different heights parallel to the substrate. The scale is adjusted to the maximum value of the complete 3D field. Although the E-field is found to be more spread out over the whole structure when looking at the response to unpolarized light, it shares with the CD response the main amplification to be located in the small tip end of the hook. Here,

the

strong

E-field

amplification

extends

several

nanometers into the surrounding medium, but shape and extend of this amplified field appear to be nearly similar for both responses. The same is true for the cross-sections at all observed heights. Those at a height of 15 nm and 20 nm allow to estimate a decay length of approximately 10 nm for both responses, suggesting that the original of the difference in sensitivity was not related to the extent of the immediate nearfield. The vertical cross-sections show that for both responses, to CD and unpolarized light, the ratio between the


27


Page 28 of 39

enhanced field in the substrate and in the free space is similar. We explored the role of the nearfield decay length on RI sensitivity via deposition of thin (1.5 nm) layers of Aluminum on top of the sample via PVD. A mild O2-plasma treatment was used to completely oxidize the Al, transforming it into 3 nm thick

Alumina

layers.59

(Al2O3)

We

performed

consecutive

addition of Al2O3 layers up to a final thickness of 12 nm added

from

the

top

of

the

structure.

CD

and

unpolarized

spectra were taken after deposition of each additional layer. Figure 5 illustrates the obtained CD and unpolarized spectra exemplarily for one hook sample (left). These experiments and comparable sensitivity

FDTD for

calculations the

CD

and

show linear

that

the

polarized

nearfield light

was

comparable indicating that the increased CD sensitivity is in the extended nearfield rather than directly at the surface of the metal at least for these high refractive index layers. The determined peak shifts (Figure 5c) are calculated from the averages of at least 4 samples per hook and handedness, though it should be noted that one set of samples was used to provide the effect of the first layer and a separate set of samples

for

the

subsequent

layers

(see

supplementary

information).


28

Page 29 of 39 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


As in the previous section, the experimental results are set in direct comparison with those from the simulated model (Figure 5df). In the simulations, the addition of the very first layer also caused unexpected peak shifts due to strong reshaping of the spectra. Since this reshaping is not observed in the experiments, we conclude that it arises from a staircasing error.60 This error comes from the pixilation of the simulation model. These pixels have sharp corners that locally enhance the electrical field way more than a smooth structure would, thus making these corners much more sensitive to changes in the local RI (see also Figure S8). To that end, we added an additional 1 nm thick Al2O3 layer in the simulations,

using

the

spectra

obtained

from

this

layer

as

reference points for the subsequent peak shifts. Furthermore, use the

simulations

to

compare

the

distribution

of

the

enhanced

nearfield a sample with and without a 12 nm thick alumina layer. As discussed before, the ratio between the enhanced field in the substrate and in the free space is similar for the two illumination cases, but in alumina sample the strongest field enhancements is located

in

the

alumina

layer.

Interestingly,

the

field

distribution for the response to unpolarized light is more affected by the alumina layers than the response to CD. Our results indicate a fundamental difference in the interaction of chiral plasmonic structures with CPL compared to unpolarized


29


Page 30 of 39

light that cannot be explained by the individual excited modes only. The CD spectra are complex with multiple modes and we propose that the bulk refractive index changes result in shifts in the dominant modes giving the possibility to measure further from the surface in the extended nearfield. While a strength of plasmonic sensors is the low detection volume given by the very rapid fall off of the near field (typically ~10 nm) which can give detection of very small amounts of material.61 This becomes a major challenge in realistic situations where the actual sensing event ends up occurring

further

from

the

surface

because

of

the

need

for

‘stealth’ non-binding layers and molecular recognition structures where

the

final

detection

event

could

then

occur

at

lower

sensitivity regions or completely outside the sensitive region. When detecting large analytes (e.g. viruses, exosomes or bacteria, which are 100 nm up to several microns in size, respectively) the bulk of the object is itself outside of the sensitive region providing

additional

challenges.

CD

based

refractive

index

measurements may provide a route to improved LSPR based sensors where detection can occur further from the surface.

4. Conclusions In

summary,

we

presented

a

comprehensive

study

on

the

potential of a simple chiral plasmonic nano-hook structure


30

Page 31 of 39 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


for

biosensing

applications

via

experiment

and

FDTD

simulations. We show that for surface attached gold nanohook structures CD based measurements have significantly higher sensitivity to bulk refractive index change (304 nm RIU-1) compared to unpolarised based measurements (146 nm RIU-1) From these results, we can conclude that the CD response of the hook is more sensitive to a wider volume compared to its response to unpolarized light but that the decay lengths of individual LSPR modes cannot readily explain this difference. We note that the RI sensitivity is close to the maximum predicted for gold structures at this spectral position where the effect of the substrate has been removed. We show that the CD sensitivity is relatively insensitive to the substrate while

as

expected

unpolarized

based

RI

sensitivity

is

significantly reduced by the presence of the substrate. This increase in sensing volume by moving from measurements with unpolarized

light

to

CD

has

interesting

potential

for

biosensing applications. It could allow the dynamic study of larger proteins and other biomolecules or make functionalized surfaces

more

sensitive.

Comparative

studies

of

CD

and

unpolarized responses could also be utilized to determine how far away from the surface a process involving local RI changes is happening. Although this study has focused on the sensing capabilities of chiral plasmonic structures, the presented


31


nano-hook

have

also

potential

use

Page 32 of 39

for

the

design

of

new

metamaterials, where chiral plasmonic structures have been shown to be useful tools for analysing and manipulating the (circular) polarization state of light.

62-65

ASSOCIATED CONTENT Supporting Information. SEM images providing additional information about the fabrication, XPS and optical spectra confirming the oxidation and reduction of the sample as well as simulations concerning the modelling of the hook structure and the near-field decay. This information is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Address: Gustav Wieds Vej 14, Aarhus C 8000, Denmark. Tel.: +45 23 38 57 89. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


32

Page 33 of 39 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


ORCID Gunnar Klös: 0000-0002-7112-8621 Matteo Miola: 0000-0002-5907-152X Duncan Sutherland: 0000-0002-5045-9915 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Independent Research Fund Denmark through grant DFF – 4184-00301.

REFERENCES (1) Brolo, A. G. Plasmonics for Future Biosensors. Nature Photonics 2012, 6 (1), 709–713. (2) Zhao, J.; Zhang, X.; Yonzon, C. R.; Haes, A. J.; Van Duyne, R. P. Localized Surface Plasmon Resonance Biosensors. Nanomedicine 2006, 1 (2), 219–228. (3) Turner, A. P. F. Biosensors: Sense and Sensibility. Chem. Soc. Rev. 2013, 42 (8), 3184–3196. (4) Lopez, G. A.; Estevez, M. C.; Soler, M.; Lechuga, L. M. Recent Advances in Nanoplasmonic Biosensors: Applications and Labon-a-Chip Integration. Nanophotonics 2017, 6 (1), 123–136. (5) Fabrizio, E. Di; Schlücker, S.; Wenger, J.; Regmi, R.; Rigneault, H.; Calafiore, G.; West, M.; Cabrini, S.; Fleischer, M.; Van Hulst, N. F.; et al. Roadmap on Biosensing and Photonics with Advanced Nano-Optical Methods. J. Opt. 2016, 18 (6), 063003. (6) Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R.; Lapthorn, A.; Kelly, S.; Barron, L.;


33


Page 34 of 39

Gadegaard, N.; Kadodwala, M. Ultrasensitive Detection and Characterization of Biomolecules Using Superchiral Fields. Nat. Nanotechnologie 2010, 5 (11), 783–787. (7) Liu, N.; Hentschel, M.; Weiss, T.; Alivisatos, A. P.; Giessen, H. Three-Dimensional Plasmon Rulers. Science 2011, 332 (6036), 1407–1410. (8) Pendry, J. B.; Schurig, D.; Smith, D. R. Controlling Electromagnetic Fields. Science 2006, 312 (5781), 1780–1782. (9) Adato, R.; Altug, H. In-Situ Ultra-Sensitive Infrared Absorption Spectroscopy of Biomolecule Interactions in Real Time with Plasmonic Nanoantennas. Nat. Commun. 2013, 4 (1), 2154. (10) Spackova, B.; Wrobel, P.; Bockova, M.; Homola, J. Optical Biosensors Based on Plasmonic Nanostructures: A Review. Proc. IEEE 2016, 104 (12), 2380–2408. (11) Lal, S.; Link, S.; Halas, N. J. Nano-Optics from Sensing to Waveguiding. Nat. Photonics 2007, 1 (11), 641–648. (12) Cooper, M. A. Optical Biosensors in Drug Discovery. Nat. Rev. Drug Discov. 2002, 1 (7), 515–528. (13) Mayergoyz, I. D.; Fredkin, D. R.; Zhang, Z. Electrostatic (plasmon) Resonances in Nanoparticles. Phys. Rev. B 2005, 72 (15), 155412. (14) Berova, N.; Nakanishi, K.; Woody, R.; Woody, R. Circular Dichroism: Principles and Applications. John Wiley & Sons 2000. (15) Fasman, G. Circular Dichroism and the Conformational Analysis of Biomolecules. Springer US 1996. (16) Johnson, W. C. Protein Secondary Structure and Circular Dichroism: A Practical Guide. Proteins Struct. Funct. Bioinforma. 1990, 7 (3), 205–214. (17) Hentschel, M.; Schäferling, M.; Duan, X.; Giessen, H.; Liu, N. Chiral Plasmonics. Science Advances 2017, 3 (5), e1602735. (18) Poulikakos, L. V.; Thureja, P.; Stollmann, A.; De Leo, E.; Norris, D. J. Chiral Light Design and Detection Inspired by Optical Antenna Theory. Nano Lett. 2018, 18 (8), 4633–4640. (19) Wang, X.; Tang, Z. Circular Dichroism Studies on Plasmonic Nanostructures. Small 2017, 13 (1), 1601115. (20) Tang, Y.; Cohen, A. E. Enhanced Enantioselectivity in Excitation of Chiral Molecules by Superchiral Light. Science 2011, 332 (6027), 333–336. (21) Frank, B.; Yin, X.; Schäferling, M.; Zhao, J.; Hein, S. M.; Braun, P. V.; Giessen, H. Large-Area 3D Chiral Plasmonic Structures. ACS Nano 2013, 7 (7), 6321–6329. (22) Karst, J.; Strohfeldt, N.; Schäferling, M.; Giessen, H.; Hentschel, M. Single Plasmonic Oligomer Chiral Spectroscopy. Adv. Opt. Mater. 2018, 6 (14), 1800087.


34

Page 35 of 39 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


(23)

Fang, Y.; Verre, R.; Shao, L.; Nordlander, P.; Käll, M. Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators. Nano Lett. 2016, 16 (8), 5183– 5190. (24) Hentschel, M.; Schäferling, M.; Weiss, T.; Liu, N.; Giessen, H. Three-Dimensional Chiral Plasmonic Oligomers. Nano Lett. 2012, 12 (5), 2542–2547. (25) Wang, Z.; Jia, H.; Yao, K.; Cai, W.; Chen, H.; Liu, Y. Circular Dichroism Metamirrors with Near-Perfect Extinction. ACS Photonics 2016, 3 (11), 2096–2101. (26) Karimullah, A. S.; Jack, C.; Tullius, R.; Rotello, V. M.; Cooke, G.; Gadegaard, N.; Barron, L. D.; Kadodwala, M. Disposable Plasmonics: Plastic Templated Plasmonic Metamaterials with Tunable Chirality. Adv. Mater. 2015, 27 (37), 5610–5616. (27) Meinzer, N.; Hendry, E.; Barnes, W. L. Probing the Chiral Nature of Electromagnetic Fields Surrounding Plasmonic Nanostructures. Phys. Rev. B - Condens. Matter Mater. Phys. 2013, 88 (4), 041407. (28) Hendry, E.; Mikhaylovskiy, R. V.; Barron, L. D.; Kadodwala, M.; Davis, T. J. Chiral Electromagnetic Fields Generated by Arrays of Nanoslits. Nano Lett. 2012, 12 (7), 3640–3644. (29) Decker, M.; Zhao, R.; Soukoulis, C. M.; Linden, S.; Wegener, M. Twisted Split-Ring-Resonator Photonic Metamaterial with Huge Optical Activity. Opt. Lett. 2010, 35 (10), 1593. (30) Ogier, R.; Fang, Y.; Svedendahl, M.; Johansson, P.; Käll, M. Macroscopic Layers of Chiral Plasmonic Nanoparticle Oligomers from Colloidal Lithography. ACS Photonics 2014, 1 (10), 1074–1081. (31) Garoli, D.; Zilio, P.; De Angelis, F.; Gorodetski, Y. Helicity Locking of Chiral Light Emitted from a Plasmonic Nanotaper. Nanoscale 2017, 9 (21), 6965–6969. (32) Song, C.; Blaber, M. G.; Zhao, G.; Zhang, P.; Fry, H. C.; Schatz, G. C.; Rosi, N. L. Tailorable Plasmonic Circular Dichroism Properties of Helical Nanoparticle Superstructures. Nano Lett. 2013, 13 (7), 3256–3261. (33) Kuzyk, A.; Schreiber, R.; Fan, Z.; Pardatscher, G.; Roller, E.-M.; Högele, A.; Simmel, F. C.; Govorov, A. O.; Liedl, T. DNA-Based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response. Nature 2012, 483 (7389), 311–314. (34) Yan, W.; Xu, L.; Xu, C.; Ma, W.; Kuang, H.; Wang, L.; Kotov, N. A. Self-Assembly of Chiral Nanoparticle Pyramids with Strong R / S Optical Activity. J. Am. Chem. Soc. 2012, 134 (36), 15114–15121.


35


Page 36 of 39

(35)

Guerreiro, J. R. L.; Frederiksen, M.; Bochenkov, V. E.; De Freitas, V.; Ferreira Sales, M. G.; Sutherland, D. S. Multifunctional Biosensor Based on Localized Surface Plasmon Resonance for Monitoring Small Molecule–Protein Interaction. ACS Nano 2014, 8 (8), 7958–7967. (36) Guerreiro, J. R. L.; Bochenkov, V. E.; Runager, K.; Aslan, H.; Dong, M.; Enghild, J. J.; De Freitas, V.; Ferreira Sales, M. G.; Sutherland, D. S. Molecular Imprinting of Complex Matrices at Localized Surface Plasmon Resonance Biosensors for Screening of Global Interactions of Polyphenols and Proteins. ACS Sensors 2016, 1 (3), 258–264. (37) Guerreiro, J. R. L.; Teixeira, N.; De Freitas, V.; Sales, M. G. F.; Sutherland, D. S. A Saliva Molecular Imprinted Localized Surface Plasmon Resonance Biosensor for Wine Astringency Estimation. Food Chem. 2017, 233, 457–466. (38) Mark, A. G.; Gibbs, J. G.; Lee, T.-C.; Fischer, P. Hybrid Nanocolloids with Programmed Three-Dimensional Shape and Material Composition. Nat. Mater. 2013, 12 (9), 802–807. (39) Verre, R.; Shao, L.; Odebo Länk, N.; Karpinski, P.; Yankovich, A. B.; Antosiewicz, T. J.; Olsson, E.; Käll, M. Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators. Adv. Mater. 2017, 29 (29), 1701352. (40) Brian, B.; Sepúlveda, B.; Alaverdyan, Y.; Lechuga, L. M.; Käll, M. Sensitivity Enhancement of Nanoplasmonic Sensors in Low Refractive Index Substrates. Opt. Express 2009, 17 (3), 2015. (41) Dmitriev, A.; Hägglund, C.; Chen, S.; Fredriksson, H.; Pakizeh, T.; Käll, M.; Sutherland, D. S. Enhanced Nanoplasmonic Optical Sensors with Reduced Substrate Effect. Nano Lett. 2008, 8 (11), 3893–3898. (42) Martinsson, E.; Otte, M. A.; Shahjamali, M. M.; Sepulveda, B.; Aili, D. Substrate Effect on the Refractive Index Sensitivity of Silver Nanoparticles. J. Phys. Chem. C 2014, 118 (42), 24680–24687. (43) Kelly, K.; Coronado, E. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107 (1), 668–677. (44) Novo, C.; Funston, A. M.; Pastoriza-Santos, I.; LizMarzán, L. M.; Mulvaney, P. Influence of the Medium Refractive Index on the Optical Properties of Single Gold Triangular Prisms on a Substrate. J. Phys. Chem. C 2008, 112 (1), 3–7. (45) Bochenkov, V. E.; Frederiksen, M.; Sutherland, D. S. Enhanced Refractive Index Sensitivity of Elevated Short-Range Ordered Nanohole Arrays in Optically Thin Plasmonic Au Films. Opt. Express 2013, 21 (12), 14763. (46) Saison-Francioso, O.; Lévêque, G.; Boukherroub, R.; Szunerits, S.; Akjouj, A. Dependence between the Refractive-


36

Page 37 of 39 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


Index Sensitivity of Metallic Nanoparticles and the Spectral Position of Their Localized Surface Plasmon Band: A Numerical and Analytical Study. J. Phys. Chem. C 2015, 119 (51), 28551– 28559. (47) Larsson, E. M.; Alegret, J.; Kǎll, M.; Sutherland, D. S. Sensing Characteristics of NIR Localized Surface Plasmon Resonances in Gold Nanorings for Application as Ultrasensitive Biosensors. Nano Lett. 2007, 7 (5), 1256–1263. (48) Valev, V. K.; Smisdom, N.; Silhanek, A. V.; De Clercq, B.; Gillijns, W.; Ameloot, M.; Moshchalkov, V. V.; Verbiest, T. Plasmonic Ratchet Wheels: Switching Circular Dichroism by Arranging Chiral Nanostructures. Nano Lett. 2009, 9 (11), 3945–3948. (49) Plum, E.; Liu, X. X.; Fedotov, V. A.; Chen, Y.; Tsai, D. P.; Zheludev, N. I. Metamaterials: Optical Activity without Chirality. Phys. Rev. Lett. 2009, 102 (11), 113902. (50) Papakostas, A.; Potts, A.; Bagnall, D. M.; Prosvirnin, S. L.; Coles, H. J.; Zheludev, N. I. Optical Manifestations of Planar Chirality. Phys. Rev. Lett. 2003, 90 (10), 4. (51) Kuwata-Gonokami, M.; Saito, N.; Ino, Y.; Kauranen, M.; Jefimovs, K.; Vallius, T.; Turunen, J.; Svirko, Y. Giant Optical Activity in Quasi-Two-Dimensional Planar Nanostructures. Phys. Rev. Lett. 2005, 95 (22), 227401. (52) Miller, M. M.; Lazarides, A. A. Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment. J. Phys. Chem. B 2005, 109 (46), 21556–21565. (53) Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B. HoleMask Colloidal Lithography. Adv. Mater. 2007, 19 (23), 4297– 4302. (54) Yesilkoy, F.; Flauraud, V.; Rüegg, M.; Al., E. 3D Nanostructures Fabricated by Advanced Stencil Lithography. Nanoscale 2016, 8 (9), 4945–4950. (55) Kontio, J. M.; Simonen, J.; Tommila, J.; Pessa, M. Arrays of Metallic Nanocones Fabricated by UV-Nanoimprint Lithography. Microelectronic Engineering 2010, 87 (9), 1711– 1715. (56) Vazquez-Mena, O.; Villanueva, L. G.; Savu, V.; Sidler, K.; Langlet, P.; Brugger, J. Analysis of the Blurring in Stencil Lithography. Nanotechnology 2009, 20 (41), 415303. (57) Kirkwood, J. G. On the Theory of Optical Rotatory Power. J. Chem. Phys. 1937, 5 (6), 479–491. (58) Condon, E. U. Theories of Optical Rotatory Power. Rev. Mod. Phys. 1937, 9 (4), 432–457. (59) Frederiksen, M.; Bochenkov, V. E.; Ogaki, R.; Sutherland, D. S. Onset of Bonding Plasmon Hybridization Preceded by Gap Modes in Dielectric Splitting of Metal Disks.


37


Page 38 of 39

Nano Lett. 2013, 13 (12), 6033–6039. Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S.; Ausman, L. K.; Atkinson, A. L.; Schatz, G. C. Methods for Describing the Electromagnetic Properties of Silver and Gold Nanoparticles. Acc. Chem. Res. 2008, 41 (12), 1710–1720. (61) Rindzevicius, T.; Alaverdyan, Y.; Dahlin, A.; Höök, F.; Sutherland, D. S.; Käll, M. Plasmonic Sensing Characteristics of Single Nanometric Holes. Nano Lett. 2005, 5 (11), 2335– 2339. (62) Rodrigues, S. P.; Cui, Y.; Lan, S.; Kang, L.; Cai, W. Metamaterials Enable Chiral-Selective Enhancement of TwoPhoton Luminescence from Quantum Emitters. Adv. Mater. 2015, 27 (6), 1124–30 (63) Li, W.; Coppens, Z. J.; Besteiro, L. V.; Wang, W.; Govorov, A. O.; Valentine, J. Circularly Polarized Light Detection with Hot Electrons in Chiral Plasmonic Metamaterials. Nat. Commun. 2015, 6, 8379 (64) Jing, L., Wang, Z.; Maturi, R.; Zheng, B.; Wang, H.; Yang, Y.; Shen, L.; Hao, R.; Yin, W.; Li, E.; Chen, H. Gradient Chiral Metamirrors for Spin‐Selective Anomalous Reflection. Laser & Photonics Reviews 2017, 11 (6), 1700115 (65) Kang, L.; Rodrigues, S. P.; Taghinejad, M.; Lan, S.; Lee, K. T.; Liu, Y., Werner, D. H.; Urbas, A.; Cai, W. Preserving Spin States Upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror Nano letters 2017, 17 (11), 7102-7109. (66) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6 (12), 4370–4379. (67) Boidin, R.; Halenkovič, T.; Nazabal, V.; Beneš, L.; Němec, P. Pulsed Laser Deposited Alumina Thin Films. Ceram. Int. 2016, 42 (1), 1177–1182. (60)


38

Page 39 of 39 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



39

Increased Refractive Index Sensitivity by Circular Dichroism Sensing

Recommend Documents