Message in a molecule

Abstract

Since ancient times, steganography, the art of concealing information, has largely relied on secret inks as a tool for hiding messages. However, as the methods for detecting these inks improved, the use of simple and accessible chemicals as a means to secure communication was practically abolished. Here, we describe a method that enables one to conceal multiple different messages within the emission spectra of a unimolecular fluorescent sensor. Similar to secret inks, this molecular-scale messaging sensor (m-SMS) can be hidden on regular paper and the messages can be encoded or decoded within seconds using common chemicals, including commercial ingredients that can be obtained in grocery stores or pharmacies. Unlike with invisible inks, however, uncovering these messages by an unauthorized user is almost impossible because they are protected by three different defence mechanisms: steganography, cryptography and by entering a password, which are used to hide, encrypt or prevent access to the information, respectively.

Figure 1. m-SMS operates as a universal sensor that can discriminate among multiple different analytes.

(a) The structure of m-SMS integrates three fluorophores: solvatochromic nile blue (A), pH-sensitive fluorescein (B) and sulforhodamine B (C), as well as distinct recognition elements, such as dipicolylamine (D), boronic acid (E), thiourea (F) and sulfonamide (G). (b) Representative emission patterns generated by m-SMS in response to different analytes or conditions. The emission was recorded in different solvents (top left) and upon adding 2 μl of an aqueous solution of metal ions* (top right, 300 mΜ) and saccharides* (middle left, 13 mM) or by changing the pH** (middle right, 0.1–0.3 M NaOH), polarity*** (bottom left, 3–9% H₂O) and upon adding commercial products* (bottom right). Initial conditions: m-SMS in *EtOH-AcOH (10 mM) and NaOH (11 mM), **EtOH-AcOH (10 mM) and ***acetonitrile (ACN). The concentration of m-SMS was 500 nM in all the solutions except for the measurements in ACN, where it was 5 μM. λ_ex=480 nm. (c) Linear discrimination analysis (LDA) of 45 representative patterns generated by different analytes under diverse conditions. Initial conditions: m-SMS in EtOH-AcOH (10 mM) and ⁱ3, ⁱⁱ6, ⁱⁱⁱ8, ^iv9 and ^v11 mM of NaOH. DMSO, dimethylsulphoxide; dsDNA, double-stranded DNA; ssDNA, single-stranded DNA; THF, tetrahydrofuran.

Figure 2. Cryptographic protection by an Enigma-like molecular cipher device.

(a) The sender converts his message to numbers by using a public alphanumeric code. (b) He then dissolves m-SMS in a chosen solution, verifies the initial emission intensity (black line) and records the emission pattern generated after adding a random chemical input (green line). The resulting intensity values, recorded every 20 nm (denoted in green letters), provide a unique encryption key. (c) The sender then encrypts the message by adding the encryption key to the original message and sends the encrypted message (cipher text) to the recipient. (d) The recipient, who possesses an identical m-SMS cipher device, repeats this procedure by setting up the correct initial state of the system (for example, solvent, sensor concentration and detector gain) and adding the same chemical x. (e) The original message is then revealed by subtracting the resulting values (green line) from the cipher text. Conditions: 500 nM m-SMS in EtOH, chemical x=NaHCO₃ (2 μl, 1 M), λ_ex=480 nm. The following illustrations were used under a license from Shutterstock.com: keyboard (credit: Alhovik), pipette (credit: extender_01) and man character (credit: Leremy).

Figure 3. Encrypting longer messages by sequentially adding chemical inputs.

(a) Encrypting a message by recording the emission spectra generated after adding NaOH (2 μl, 0.2 M, red letters) and then after adding CuCl₂ (2 μl, 0.3 mM, blue letters) to 500 nM SMS in EtOH-AcOH (10 mM). (b) Encrypting the same message by recording the emission spectra after adding NaOH (2 μl, 0.35 M, red letters ) and then GenTeal eyedrop (2 μl, blue letters) to 500 nM SMS in EtOH-AcOH (10 mM). (c) Encrypting the same message by using a single, broad emission spectrum obtained after adding NaOH (0.5 μl, 0.35 M) and CuCl₂ (1 μl, 0.3 mM) to 5 μM SMS in acetonitrile. These experiments (a–c) also demonstrate how the same message can be differently encrypted by changing the chemical inputs (a versus b) or by changing the initial state of the system (a versus c). (d) Representative messages that were successfully decrypted by untrained, randomly selected users. Initial conditions: m-SMS (500 nM) in *EtOH, **EtOH-AcOH (10 mM) and NaOH (6 mM), and ***EtOH-AcOH (10 mM) and NaOH (10 mM).

Figure 4. Password protection by generating sequence-dependent encryption keys.

By appropriately choosing chemical inputs, m-SMS can operate as a molecular keypad lock that generates the correct encryption/decryption keys (emission patterns) only when the chemical inputs are introduced in the right order. (a) Different encryption keys generated by introducing the four possible combinations of two-digit chemical ‘passwords' consisting of ZnCl₂ (1) and Na₃PO₄ (2) as inputs signals. (b) LDA mapping of the encryption keys generated in response to the 27 possible combinations of three-digit chemical passwords, where ZnCl₂ (1), Na₃PO₄ (2) and NaOH (3) serve as input signals. The clusters corresponding to the nine unique encryption keys are denoted in circles. Conditions: each digit corresponds to the addition of 2 μl of 1 (0.08 M), 2 (0.08 M) or 3 (0.1 M) to 60 μl m-SMS (500 nM) in EtOH. (c) Text obtained by decrypting the cipher text with the correct password (331) and by the other eight unique combinations.

Figure 5. Steganographic protection by hiding m-SMS on plain letter paper.

(a) 1.1 μl of m-SMS (440 μM) was hidden on a random spot within the logo of the Weizmann Institute and the letter was sent to a recipient by regular mail. Note that the text within this letter does not contain any valuable information. (b) The recipient, who obtained the cipher text and knows the initial conditions, extracts m-SMS from the paper by incubating the logo in 1 ml of EtOH-AcOH (10 mM). (c) To uncover the message, the receiver adjusts the correct concentration of m-SMS by calibrating its initial emission intensity (top) and generates the decryption key by recording the emission pattern following the addition of each chemical input (inputs 1–3). (d) The resulting text is a message that was encrypted by the Enigma machine. The letter colours correspond to relevant decryption keys shown in c. Conditions: 1 μl of (1) NiCl₂ (0.15 M), (2) KOH (2.5 M) and (3) Na₄EDTA (0.27 M) were sequentially added to a 60-μl solution of m-SMS (500 nM) in EtOH-AcOH (10 mM). The hand-writing text image (credit: amiloslava) is taken with permission from Shutterstock.com.

Figure 6. Versatility of the m-SMS technology.

Secret communication was achieved by using (a) a hand-held spectrometer, and (b) a second molecular cipher device (m-SMS₂) integrating coumarin (A), fluorescein (B) and a cyclen ligand (C). (c) Encryption patterns generated by m-SMS (blue lines) or m-SMS₂ (black lines) under the same conditions. The emission of each sensor (250 nM) was recorded in EtOH solution containing NaOAc (1 mM) and ZnCl₂ (1.3 mM; dashed line) and after adding AcOH (16 mM; solid line). m-SMS and m-SMS₂ were excited at 480 and 420 nm, respectively.