# Advanced Secure Information Retrieval Technology for Multilayer Information Extraction

Authors: Shoude Chang; Kui Yu; Jiaren Liu
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Secure information retrieval technology aims at status identification and documentation authentication. Ideally, materials or devices used in these technologies should be hard to find, difficult to counterfeit, and as simple as possible. This manuscript addresses a novel information retrieval technology, with photoluminescent (PL) semiconductor quantum dots (QDs) synthesized via wet chemistry approaches used as its coding materials. Conceptually, these QDs are designed to exhibit emission at Fraunhofer line positions, namely, black lines in the solar spectrum; thus, the retrieval system can extract useful information under sunshine covering areas. Furthermore, multiphoton excitation (MPE) technology enables the retrieval system to be multilayer information extraction, with thin films consisting of QDs applied to various substrates, such as military helmets and vehicle and fingernails. Anticipated applications include security, military, and law enforcement. QD-based security information can be easily destroyed by preset expiration in the presence of timing agents.


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## Body

## 1. Introduction

There are two important issues in security
technologies: information storage and retrieval. The visibility of the printed
pattern of a barcode is vulnerable to counterfeiting; therefore, the most
prevailing barcode technology is definitely not qualified in many security
applications. Regarding high-level security, a hidden information carrier which
is invisible to human eyes and is tiny in size is mandatory; moreover, the invariance of the information encoded
by the carrier is critical to simplify the information retrieval procedure with
enhanced reliability. For example, the information encoded including sequence- or pixel-based
classification does not change with the position or rotation of its host object.Multiplexed spectral coding
technology that makes use of multiple wavelengths and multiple intensities as
the coding space meets the invariant requirement for security, to a certain
degree. Organic dyes and metal complexes are commonly used as fluorescence-sensing
materials in various applications. Basically, they seem to be suitable in the
multiplexed spectral coding technology; however, their intrinsic optical
properties limit them to be ideal candidates. For example, different dye
molecules require different excitation wavelengths; furthermore, it is
different to retrieve information from a mixture of these fluorescent molecules
due to their broad emission bandwidth and asymmetry. The limitation may also be
related to the certain reaction of different dye molecules or the immiscibility
of some of the dye molecules in a common matrix material. In order to meet the
prerequisite for multiplexed encoding and decoding, the multiplexed spectral
coding technology demands a set of luminescent materials with required
properties, such as (1) single-light source for all materials to emit at
different wavelength; (2) each emission independent of the excitation
wavelength; (3) stable emission with narrow bandwidth from each luminescent material;
(4) no chemical interaction among the fluorescent molecules; (5) no emission from
matrix materials; (6) good miscibility of all luminescent materials in the
selected matrix materials.Recently colloidal photoluminescent (PL) semiconductor quantum dots (QDs) have
demonstrated many of the above-mentioned characteristics. For example, QDs
exhibit emission with narrower bandwidth, broader absorption, and better photostability,
compared to traditional luminescent materials. Furthermore, they are excellent
with multiphoton excitation due to large multiphoton action cross-sections;
accordingly, they are magnificent candidates for multilayer information
extraction. Moreover, their bandgap emission and absorption peak positions can
be easily and accurately tuned via the control of their size, structure, and composition.
Due to their intrinsic optical properties, QDs are suitable in the multiplexed
optical coding technology with potential in security and defence [1–5]. Basically,
QDs are ultrasmall nanocrystals and are spherical in shape, usually in the
range of 1–10 nm. Elements made up of QDs are often from Group IIB and Group VIAin the periodic table. QDs can be binary
compounds, such as cadmium selenide (CdSe) and zinc sulfide (ZnS), or ternary
compounds such as CdTeSe and ZnCdS, or layered structures such as core/shell
CdSe/ZnS and CdTe/CdSe/CdTe [6–12]. When these nanocrystals dispersed in a
transparent solution are excited, they provide coding information based on
their emission position and intensity; such a solution with the secure information
encoded is the so-called infoink. A mixture of various QDs with different
emission positions can be dedicatedly designed to feature a special code with a
set of data consisting of the emission positions and intensity. Such coding information
is hidden in a fluorescence spectrum; accordingly, a spectroscopic device is needed
rather than a scanner or camera to decode the encoded information. It is
necessary to point out that such QD-based devices feature miniature in size and
invisible to human eyes, demonstrating their potential in security applications.Chan et al. proposed
the idea of using colloidal QDs in multiplex bioimaging and biolabeling [13];
such a multiplex detection only involved single-photon excitation in solution.
Bawendi and Jensen in MIT reported the design of using
QDs for inventory control [14, 15]; however, no practical approaches were
provided about their information retrieval system. Also, we demonstrated a
QD-based information retrieval system with patents
granted [16, 17]. In this manuscript, we describe, in detail, how to
expand our patented technology to higher dimensional coding space, with its
feasibility in sunlight-covering areas. In addition, we present the details of our
advanced information retrieval system with QDs as the coding materials; a
prototype system is acknowledged to extract information from an ID card.


## 2. Infoinks Consisting of QDs

Colloidal photoluminescent QDs as “infoinks” for biolabeling
were proposed about ten years ago [18–20]; meanwhile, significant advances in solution-chemistry
synthesis expand such applications of QD-based infoinks to many areas including
security [9, 14–17, 21, 22]. Various photoluminescent QDs with different emission
wavelengths provide a great number of combinations of wavelength and intensity.
For example, an encoder using 6-wavelength and 10-intensity schemes has a theoretical
coding capacity of about one million discrimination codes. The coding capacity
can be even expanded by utilizing a third parameter, such as 1D sequence or 2D
array of QDs. To be able to use QDs for the spectral coding of nonbiological
objects such as banknotes, passports, certificates, and other valuable documents,
paintable or printable QD/polymer/solvent infoinks are needed. The infoinks,
consisting of different QDs, polymers, solvents, and additives, can be
digitally printed onto various surfaces for coding purposes. A hybrid
optic-electronic-digital system is used to extract the data from the resulting
emission spectra. The detailed description is given in Section 3.After the
infoink is applied to a target surface and dried, the polymer becomes matrix
materials, in which the mixture of different QDs with predesigned emission
features is dispersed homogeneously [23]. The polymer used should not have
quenching effects on the fluorescence of the QDs, and should satisfy other
requirements, such as reasonable solubility in the selected solvent, long-term
environmental stability, and good compatibility and miscibility with the QDs.CdSe nanocrystals in the nanometer-sized
range can be well dispersed in toluene, which is also a good solvent for
polystyrene (PS). Therefore, the infoink prepared in our preliminary work
consisted of CdSe nanocrystals (Evident Technologies [24]), high molecular
weight PS, and toluene (both from Aldrich, St.Louis, Mo, USA). According to their individual emission intensity, the CdSe QD
ensembles were mixed with different ratios, together with a certain amount of
PS and toluene to engineer the infoinks with required emission spectral
features and viscosity for testing. Furthermore, inhouse-synthesized CdSe-based
nanocrystals and CdS nanocrystals were used in the present study [25–29]. The
inhouse CdSe QDs were synthesized with a slow growth rate for high-quality and
large-scale production. This nonorganometallic approach involved the addition
of a solution with a chalcogen source in tri-n-octylphosphine (TOP) to a
solution of CdO in TOP at one temperature, with subsequent growth at a lower
temperature. A slow crystal growth rate was achieved with the zero growth rate
accomplished via tuning the Cd-to-Se precursor molar ratios, as assessed by the
temporal evolution of the optical properties of the growing nanocrystals
dispersed in nonpolar hexanes (Hex) and polar tetrahydrofuran (THF). It has
been acknowledged that a higher particle growth rate results in greater surface
roughness, more surface defects, and lower PL efficiency. An approach for
removing the surface defects is to control a small growth rate, ideally to achieve
a zero growth rate, which means that the average rate of removal of atoms from
the nanocrystal surface is equal to that of addition of atoms to the nanocrystal
surface. The nanoparticles with the size close to a zero growth rate in an
ensemble possess the smoothest and defect-free surfaces and highest PL
efficiency. In general, the inhouse synthetic approach is excellent in terms of
the control of the growth rate for the CdSe nanocrystals with high surface
quality, a simple system but a rational choice for both fundamental understanding
and tailoring applications. Also, such slow size growth is essential for
large-scale production as well as for further in situ modification such as for
core/shell materials. It is necessary to point out that the zero growth rate
methodology leads to our magic-sized QD ensembles, the latest in our synthetic
laboratories; these single-sized QD ensembles exhibit bandgap emission with
bandwidth as narrow as 8 nm are ideal for infoinks for security applications
[9, 25, 26].


## 3. Information Coding and Retrieval

A 2D coding space could be spanned by the wavelengths and
intensities of the mixed PL-QDs. Figure1 illustrates, schematically, the designed infoink samples
consisting of three different QDs with different emission wavelengths.
Adjusting the various ratios of the three QDs can produce a series of 3 digital
codes. The infoinks consisting of polymer, solvent,
multiple QDs, and other additive are prepared and putting on the objects that
need to be coded. A hybrid optic-electronic-digital system is used to extract
the data. It is well acknowledged that one QD ensemble has a very broad absorption
spectrum, with its single emission position (in wavelength, λem) independent of excitation wavelengths (λex)
as long as λex
is shorter than the excitation threshold (the first absorption peak) [5]. Accordingly,
we proposed a new concept on using mixtures of multiple single-color QDs to
create highly secret cryptograms based on their absorption/emission properties
[30], in addition to the optical coding based on fluorescence of semiconductor
quantum dots (QDs). The key to the readout of the optical codes is a group of
excitation lights with the predetermined wavelengths programmed in a secret
manner. The cryptograms can be printed on the surfaces of different objects
such as valuable documents for security purposes.Figure 1
Examples of
multiplex codes provided by the infoinks prepared with three different QDs
exhibiting red, green, and blue emissions.Figure2 shows a group of emission spectra collected from a mixture of three
single-color QDs in toluene excited at a λex ranging from 350 to 510 nm with a constant interval of 10 nm. The excitation peaks (λex=510, 500, 490, 480, 470, 460, and 450 nm) and three diffraction-induced
second-order peaks (from λex=350, 360, and 370 nm) are also
recorded in the measurement range. The inset illustrates two sets of selected
spectral data normalized to a 10-level (0–9) intensity scale.Figure 2
Emission
spectra of a mixture of three single-color QDs with different excitations.It is
easy to understand that anλex-dependent
spectral change can be simply used to increase the coding capacity. Assuming that a
10-level (0–9) intensity scale is used for coding, a two-color
QD system under the excitation of a single λex can theoretically generate 99 codes,
including those involving the fluorescence intensity level of 0. If multiple λex elements can be applied to the
system, the coding capacity is theoretically expanded to a multifold of 99.
This should well accommodate all Latin characters, with a large redundancy. In
the same way, thousands of the most frequently used Chinese characters could be
coded with a tricolor QD system plus multiple λex elements. Of course, from a practical point of view on the available single-color QD
ensembles exhibiting narrow bandwidth and distinguishable intensity,
using more spots spatially arranged in a particular way to represent a character
could be a better way to increase the coding capacity, though at the cost of
reducing the readout speed.The coding capability can be furthermore extended from 2D surface to
multiple layers by using two-photon exciting and 3D information extraction. The
process of two-photon excitation is currently being examined for a variety of
applications, for example, ultrahigh-density optical data storage, biochemical
imaging, and 3D microfabrication [31]. We have
investigated the two-photon excitation characteristics of QDs, and have begun
to explore their application for 3D information extraction. Theoretically, when
laser light at a wavelength λp
is tightly focused inside transparent QD materials so that its intensity in a
small volume around the focus surpasses specific threshold, two-photon
absorption is triggered if λp>λem>(λp/2), where λem
is denoted as any of fluorescent emission wavelengths. Because the emission is localized in
a part of the focal volume, 3D information could easily be extracted with the
micron or submicron spatial resolution depending on focal optics and laser
intensity. Experimentally, we demonstrated two-photon
excitation based on CdSe, CdS, and/or CdSe/ZnS QDs, and an 800 nm femtosecond
laser with the repetition rate of 78 MHz and the pulse duration of 120–150 femtoseconds.
Figure 3 shows a typical image with one red spot, obtained from a CdSe QD
dispersion by two-photon excitation. The volume of the focused spot can be
manipulated by using various focusing objectives, laser beam diameter, and
laser intensity. The emission and absorption spectra of one CdSe QD ensemble
and one CdSe/ZnS QD ensemble are shown in the parts a and b of Figure 4,
respectively, the emission spectra were collected with 800 nm two-photon
excitation.Figure 3
A typical image
with one red spot taken from a CdSe QD ensemble dispersed in hexanes with
two-photon excitation.Emission (thick
line, left axis) spectra and absorption (thin line, rightY-axis) from (a) one
CdSe ensemble and (b) one CdSe/ZnS ensemble. The emission spectra were obtained
with 800 nm two-photon excitation.
(a)(b)Obviously, the two fluorescence spectra by
two-photon excitation shown in Figure4 have little signal-to-noise ratios, with
unambiguous recognition of fluorescence peaks and shapes. Moreover, the
relationship between the emission intensity and the two-photon excitation pump
intensity was investigated, with one CdS QD ensemble exhibiting an emission peak around
470 nm. While the fluorescent spectra of the CdS QD ensemble pumped by
different pump power were shown in Figure 5(a), the fluorescent intensity
versus the pump power shown in Figure 5(b) was calculated as y=0.8515x1.4016. Such a relationship
does not agree with that of theoretical prediction y=α⋅x2 under the coherently driven two-photon
resonant absorption. The discrepancy could be explained by the detuning of the
470 nm emission peak from the coherent two-photon pumping at around 800 nm which
is equal to 3723 cm−1[32].(a) The
fluorescent spectra of CdS QDs pumped by various two-photon excitation powers.  (b) The relation between the CdS fluorescence
intensity (Y-axis) and pump power intensity (X-axis) obtained.
(a)(b)We reported a
2D information retrieving system [33],
consisting of an exciting light, a spectrum sensor, and a signal-processing
unit. Figure 6 shows a prototype of such a
system for retrieving the information hidden in a tiny spot of a card. The exciting
light is provided by a 370 nm LED light source. An optical fiber bundle guides
the exciting light to the infoink spot applied on the surface of any object. The
fluorescence emitted from the QDs is collected by the detecting fiber in the
fiber bunch and fed to a spectrometer. The data generated by the spectrometer
is further delivered to an intelligent instrument, namely, a microprocessor or
a PC, which eventually extracts the information originally coded in the infoink.
In order to obtain an even exciting light, the exciting fibers are arranged to
surround the detecting fiber evenly to form an optical fiber bundle, as shown
in Figure 6.  A rubber cup is connected
at the end of the fiber bundle to ensure that only the excited fluorescent
light can enter the detecting fiber.Figure 6
Our prototype of
the information retrieving system designed for an ID card.The raw data
collected from spectrometer is processed by a computer with the following
steps: (1) removing the noise by a digital filter; (2) separating the spectral
center lines emitted by infoinks from the overlapped spectra; (3) finding the
wavelengths (Ws) and intensities (Is) of all the spectral center lines; (4)
calibrating these Ws and Is, and retrieving the original data
according to a prior known codebook.Figure7
shows a fluorescence spectrum measured from an
infoink containing only one single-color QD ensemble with emission peak at 535 nm. Because of the Gaussian-like feature of the emission peak shape with a
certain bandwidth, it is easy to understand that neighboring emission spectral
profiles can mutually affect each other, including their emission intensity and
peak position, when an infoink consisting of several single-color QD ensembles emitting
at different positions. Figure 8 shows one example of such
a spectral alias. The acquired emission spectrum from one infoink is the
top/thick black curve; this infoink consists of two single-color QD ensembles,
one is QDs 611 nm, the other is QDs 632 nm. As shown in Figure 8, the input
spectrum can be deconvoluted into two spectra: one is with
intensity of 1.0 (the gray curve), the other 0.2 (the thin black curve); these
two deconvoluted spectra are resulted from the two QD ensembles. As the
intensity of the spectrum represented by the thin black curve is only about 1/5
of that of the spectrum represented by the gray curve, the peak of the former could
not be distinguished from the input spectrum. Such an ambiguous determination
of the peak position will eventually result in a decoding error if no deconvolution
is performed.Figure 7
One
photoluminescent spectrum from an infoink consisting of one single-color QD
ensemble.Figure 8
Illustration of the importance of spectrum
deconvolution for spectral line extraction during decoding.Let us turn
our attention to spectrum deconvolution. A spectrum function of one infoink can
be described as(1)f(λ)=∑i=1Nki⋅δ(λ−λi)⊗p(λi), where δ(λ) represents an impulse function, physically, a spectral line. ki is the
intensity of a δ(λ) at λi.p(λi) denotes the profile function centered at λi.⊗ represents
a convolution operation. As described above, the bandwidth of an emission
profile of the infoink is the main reason for the spectrum alias. To get rid of
the alias effect, the spectrum line must be separated by means of deconvolution
operation. Let F(u) and D(u) be Fourier
transforms of f(λ) and δ(λ), respectively, provided
all the p(λi)s have the same Fourier
transformation P(u), the separated
spectrum line at λm
is obtained by the inverse
Fourier transform (IFT) of (1):(2)IFT[F(u)P(u)]=∑i=1Nkiδ(λ−λi). Equation (2)
yields a serial of δ(λ)s, indicating that all the spectral lines are
extracted and separated as individual impulses. However, as each spectrum
profile of infoink is actually different from others, the deconvolution
operation can only extract one narrow sharp impulse. To find all spectral
lines, kiδ(λ−λi),i=1⋯N,N times operations are needed.For 3D
information extraction using two-photon excitation, depth scanning devices are required,
collaborated with a 2D extraction system. Controlled by a computer, the
information encoded in layers will be retrieved layer by layer.When applying QD-based labeling technology to sun-covering
areas, there is a critical issue to be addressed: the brightness
of sunlight may overpower most optics-based solutions.
Specifically, detecting a return fluorescent signal can be very difficult as
the return optical signal can get overwhelmed by sunlight. Fortunately, the sun is not a
perfectly continuous white source. There are many black lines seen in the solar
spectrum—the Fraunhofer lines. As shown in Figure9, the solar spectrum has
been split into pieces, from the blue (left) end to the red (right) end by
these absorption lines, literally thousands, in fact, if one looks in fine
enough detail. These black spectrum lines provide the
coding space. Subsequently, the infoink will be engineered with QDs
whose emission positions are located in those black lines. Although the encoding information provided by these
wavelength positions could not be covered by the reflection of the powerful
daylight, in our retrieving device proposed and patented, special
filters are still used to extract the useful wavelengths, namely, Fraunhofer line
signals while screen the others.Figure 9
Fraunhofer
lines in the solar spectrum.


## 4. Applications

A direct application is the ID card identification. In
order to code the documents automatically and fast, a principal ink should be
prepared first. The principal ink is an infoink that has a unique fluorescent
wavelength. For an application using 6 wavelengths, 6 principal infoinks are
needed.  Controlled by a computer, these
principal infoinks are mixed to form a coded infoink and then applied onto the
surface of an ID card. As the last step, a cover layer is applied to the top of
infodrop to prevent the infodrop from being scratched or damaged by any means. The
polymeric material used for the protective layer can be the same as or
different from that used in infoinks.An
example of our retrieving system is shown in Figure10. The implementation of
this system is shown in Figure 6. It is true that such a tiny, transparent
information carrier can be easily applied to the hard/soft surface of an
object, including the passport, documents, vehicle, and even a nail of human
finger.Figure 10
One system of our retrieving system used for ID
card identification.Another
potential application is proposed for friend/enemy discrimination. The problem
of identifying fires friendly from unfriendly forces
becomes critical in the modern battles. The rise of the so-called “smart weapons”
has enhanced the accuracy of ordnance delivery, but, sadly, has also increased
the casualty rates of the so-called “friendly fire” incidents. After World War
II,
the use of
precision weapons and advances in intelligence technologies for air and space
has drastically revolutionized air warfare; meanwhile, the friendly fire casualty
rate increased. The percent of casualties from 21% in World War II changes to
39% in Vietnam War and 49% in Persian War. (This percent was estimated by the American
War Library on friendly fire casualties, both fatal and nonfatal, based on
historic War Department, Department of the Navy, and Department of Defense
casualty reports detailing various battle reports).After Desert
Storm, the Pentagon conducted extensive researches into developing ways for
friend/enemy discrimination, but suspended many of them due to cost. One
program—called the
Battlefield Combat Identification System or BCIS—had been
introduced to prevent friendly fire. However, to equip the Army with such
automatic identification systems would have cost as much as $40,000 per vehicle. Up to date, the friendly fire is still playing a role as a critical
“enemy” in the battlefield. One possible solution is the use of QD-based
information coding and retrieving system. Instead of using expensively active
radar technology, the QD-based system takes the advantages of the passive
fluorescent features of QDs. To avoid friendly fire from aircraft attacking,
the coded infoink is painted on the top surfaces of vehicles and soldiers
helmets as their ID labels. By sending a probing beam and detecting the reflected spectrum, the aircraft could distinguish
these labels before firing a missile or bomber. Even after the firing, the
intelligent device built in the warhead could
disable the deadly exploding if the ID signal is detected as friend army. The
more soldiers and vehicles are involved in a battlefield, the higher level ID
labels are demanded to enhance the discrimination and reduce a large-scaled
casualty caused by friendly fire.


## 5. Summary and Conclusion

The
present manuscript argues about basic ideas with preliminary experimental
results about the use of semiconductor QDs for security and military applications.
Photoluminescent QDs are suitable for security identification with, due to
their intrinsic optical properties, such as narrow emission and broad
absorption, good photostability, as well as easy tuning of their photoemission
peak positions via the control of their size, structure, and composition. Thus,
QDs satisfy the main requirements in security identification, such as easy
encoding and decoding, large coding space, invisible, and translation-rotation
invariant. Basically, our QD-based information retrieval technology has bright
future in many applications; however, it is not a mature technology. Additional
experiments, such as sensitivity to nonuniform illumination, detection
limitations, repeatability, and data collecting time, will be conducted in our
future work.Finally, we would like to discuss three critical factors
for successful information retrieval, namely, an advanced methodology for multilayer
information retrieval using QD-based infoinks with specially designed emission properties.Factor one:
long-term stability of QDs. The emission stability of
QDs has been monitored for weeks. We are still making efforts on testing QD-based
materials for longer stability. Different synthetic approaches lead to QDs with
different surface properties and thus optical properties; Figure11 shows the
comparison of the photostability of the QDs developed by us (NRC-SIMS) and by
Evident Technologies [24–26]. Recently developed magic-sized QD ensembles are very stable
and should be excellent candidates [9].Emission spectra for the comparison of the photostability after
UV-irradiation of CdSe QD ensembles from (a) SIMS-NRC and from (b) Evident
Technologies. The two CdSe ensembles were dispersed in toluene with a similar
concentration. The UV source is a 365 nm lamp, and the irradiation hours are
indicated by the different color. NRC-QD ensemble exhibited little blue shift
during UV-irradiation, as compared to that of Evident Technologies QD
ensemble.
(a)(b)Factor two:
detection from a distance. At least, three factors
are involved: (1) the quality/sensitivity of sensors, (2) the exciting beam, and
(3) the QD quality including emission intensity and bandwidth. The first two
are related to hardware development. Investigation will be carried out on
concentrating the infoinks to enhance the fluorescent signals.Factor three:
the algorithm used for the deconvolution of an input spectrum
plays an important
role in information retrieval. The deconvolution procedure is adopted basically
due to its ability of narrowing and separating the overlapped neighboring QD-emitting
spectra. However, if one QD ensemble exhibits an emission spectrum with changes
in its shape, errors may be introduced during deconvolution. Fortunately, QD-based
infoinks have exhibited little changes in their individual emission shapes.
Although many algorithms of signal detection and spectrum evaluation are
available [34], they are basically designed for the purpose of improving the
signal-to-noise ratio. For the present technology with QD-based emission, the algorithm
for spectrum deconvolution is more needed. When a useful signal in the input is weak, a
calibrating procedure should be introduced.



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